Ecology and Ecosystems Analysis 3031452585, 9783031452581

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Christopher S. Cronan

Ecology and Ecosystems Analysis

Ecology and Ecosystems Analysis

Christopher S. Cronan

Ecology and Ecosystems Analysis

Christopher S. Cronan School of Biology and Ecology University of Maine Orono, ME, USA

ISBN 978-3-031-45258-1    ISBN 978-3-031-45259-8 (eBook) https://doi.org/10.1007/978-3-031-45259-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023, Corrected Publication 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Paper in this product is recyclable.

Acknowledgments

I have been privileged to cross paths and to collaborate with an amazing group of scientists, mentors, and colleagues who have influenced and inspired my journey in ecosystems ecology. At the University of Pennsylvania, I worked with and took classes from Michael Levin, Bob Giegengack, Ruth Patrick, and Ian McHarg, and also conducted research at the Philadelphia Academy of Natural Sciences under Ruth Patrick. During a summer at the Marine Biological Lab, I was enriched by lecture and lab sessions with Holger Jannasch, Eugene Odum, and E. O. Wilson. In the doctoral program at Dartmouth College, Bill Reiners and Bob Reynolds provided excellent mentoring, and I enjoyed rich interactions with colleagues Peter Vitousek, Peter White, Gary Lovett, and Dale Johnson (at UW in Seattle). In the course of the interdisciplinary ILWAS, RILWAS, and ALBIOS research programs, I shvared the excitement of scientific discovery with Steve Gherini, Bob Newton, Rich April, Charlie Driscoll, Gray Henderson, Dave Schindler, Michael Hauhs, Egbert Matzner, Nils Christophersen, Dudley Raynal, and Mike Kelly. I also enjoyed many fruitful interactions with Alan White at the University of Maine and Keith Killingbeck at the University of Rhode Island. At the end of every day or week, it was always a joy and a delight to come home to my amazing and supportive wife Karen, daughter Chelsey, and son Matt. This book is dedicated to them with love and deep appreciation.

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Contents

1

Introduction����������������������������������������������������������������������������������������������    1

2

Life Processes�������������������������������������������������������������������������������������������    5 2.1 Photosynthesis and Plant Primary Production����������������������������������    6 2.2 Unlocking Metabolic Energy with Cellular Respiration������������������   10 2.3 Energy Relations and Carbon Allocation������������������������������������������   12 2.4 Water Relations ��������������������������������������������������������������������������������   15 2.5 Osmoregulation��������������������������������������������������������������������������������   17 2.6 Gas Exchange������������������������������������������������������������������������������������   18 2.7 Heat Balance and Thermal Regulation ��������������������������������������������   19 2.8 Nutrient Relations ����������������������������������������������������������������������������   21

3

Environmental Analysis��������������������������������������������������������������������������   23 3.1 Environmental Gradients������������������������������������������������������������������   24 3.1.1 An Example of a Montane Gradient in the Southwestern U.S��������������������������������������������������������   25 3.2 Geographic Ranges of Species in Relation to Environmental Factors������������������������������������������������������������������   26 3.3 Patterns of Light and Solar Radiation����������������������������������������������   27 3.4 Climatic Patterns – Temperature and Moisture��������������������������������   28 3.5 A Case Study Exploring the Relationship of Plant Diversity and Environmental Influences����������������������������������������������������������   31 3.6 Influence of Geologic Factors in the Environment ��������������������������   32 3.6.1 Surficial and Bedrock Geology��������������������������������������������   33 3.7 Influence of Soils in the Environment����������������������������������������������   36 3.7.1 Soil Site Quality��������������������������������������������������������������������   37

4

Population Ecology����������������������������������������������������������������������������������   45 4.1 Population Growth and Regulation��������������������������������������������������   46 4.2 Trade-Offs in Reproductive Effort����������������������������������������������������   50 4.3 Population Mortality Patterns ����������������������������������������������������������   51 4.4 Case Study – The Population Ecology of Eastern Coyote����������������   51 vii

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4.5 A Method for Estimating the Size of Wild Populations ������������������   53 4.6 Population Life Tables����������������������������������������������������������������������   54 4.7 Life History Strategies����������������������������������������������������������������������   55 4.8 Evolutionary Processes ��������������������������������������������������������������������   56 4.8.1 Time Scales of Evolution������������������������������������������������������   59 4.8.2 The Origin of Species ����������������������������������������������������������   60 4.9 Behavioral Ecology of Animal Species��������������������������������������������   61 5

Community Ecology��������������������������������������������������������������������������������   65 5.1 Historical Perspectives on Communities������������������������������������������   65 5.2 Ecological Interactions in Communities������������������������������������������   66 5.3 Competition��������������������������������������������������������������������������������������   69 5.3.1 A Simple Mathematical Model for the Effects of Competition on Population Growth����������������������������������   71 5.4 Trophic Relationships and Predation������������������������������������������������   72 5.5 Analysis of Food Webs with Stable Isotopes������������������������������������   76

6

 Landscape Ecology and Conservation Biology ������������������������������������   81 6.1 Concepts of Conservation Biology ��������������������������������������������������   82 6.2 Concepts of Landscape Ecology and Land Use Change������������������   84 6.2.1 Rank Your Personal Conservation Priorities ������������������������   88 6.3 Designing an Action Plan for Conservation Biology������������������������   88 6.4 Moving On����������������������������������������������������������������������������������������   92

7

Forest Ecosystems������������������������������������������������������������������������������������   93 7.1 A Geographic Perspective����������������������������������������������������������������   93 7.2 History of the Forest Landscape ������������������������������������������������������   94 7.3 Discovering Secrets of the Forest ����������������������������������������������������   95 7.4 Using Metrics to Quantify Forest Conditions����������������������������������   97 7.5 Consumers in a Forest Community��������������������������������������������������  100 7.6 Avian Ecology in a Forest Ecosystem����������������������������������������������  102 7.7 Life History Patterns in a Forest Ecosystem������������������������������������  104 7.8 Primary Production and Energy Flow in Forest Ecosystems������������  105 7.9 Hydrologic Processes in a Forest Ecosystem������������������������������������  108 7.9.1 Precipitation��������������������������������������������������������������������������  109 7.9.2 Stream Runoff����������������������������������������������������������������������  109 7.9.3 Evapotranspiration����������������������������������������������������������������  110 7.9.4 Water Storage������������������������������������������������������������������������  110 7.9.5 Application of a Water Budget����������������������������������������������  110 7.10 Nutrient Cycling��������������������������������������������������������������������������������  112 7.11 Disturbance and Succession��������������������������������������������������������������  116

8

Lake Ecosystems��������������������������������������������������������������������������������������  123 8.1 Environmental Characteristics of Lakes ������������������������������������������  123 8.1.1 Trophic Conditions ��������������������������������������������������������������  125 8.2 Lake Structure and Habitat Zonation������������������������������������������������  126 8.3 Seasonal Cycles and Lake Stratification ������������������������������������������  128

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8.4 Lake Biota and Food Webs in a Lake Ecosystem ����������������������������  131 8.5 Successional Changes in Lakes��������������������������������������������������������  134 8.6 Trophic Cascades and Biomanipulation ������������������������������������������  135 8.7 Primary Production and Energy Flow in Lake Ecosystems��������������  136 8.8 Nutrient Cycling in Lake Ecosystems����������������������������������������������  138 8.9 Effects of Disturbance and Stress on Lake Ecosystems ������������������  140 8.9.1 Lake Acidification Stress������������������������������������������������������  140 8.9.2 Introduction of an Exotic Non-native Fish Predator������������  142 9

Stream Ecosystems����������������������������������������������������������������������������������  143 9.1 Environmental Conditions in Stream Ecosystems����������������������������  143 9.1.1 Stream Drainage Network����������������������������������������������������  143 9.1.2 Stream Hydrology����������������������������������������������������������������  144 9.1.3 Physical-Chemical Parameters Affecting Organisms ����������  145 9.1.4 Streams and the Surrounding Drainage Basin����������������������  148 9.1.5 Habitat Structure and Zonation��������������������������������������������  150 9.2 Stream Communities and Food Webs ����������������������������������������������  152 9.2.1 Producers and Consumers����������������������������������������������������  152 9.2.2 Food Webs����������������������������������������������������������������������������  153 9.3 Ecosystem Processes in Streams and Rivers������������������������������������  155 9.3.1 Energy Flow Patterns������������������������������������������������������������  155 9.3.2 A Novel Twist on Nutrient Cycling in Stream Ecosystems ����������������������������������������������������������  156 9.4 Ecological Effects of Stream Disturbance and Stress����������������������  157 9.4.1 A Case Study of Watershed Urbanization����������������������������  158 9.4.2 A Case Study of Long-Range Land Use Impacts ����������������  159

10 Wetland Ecosystems��������������������������������������������������������������������������������  161 10.1 Overview of Wetlands��������������������������������������������������������������������  162 10.2 Types of Wetlands ��������������������������������������������������������������������������  163 10.3 Patterns and Processes in a Northern Bog Ecosystem��������������������  165 10.3.1 Environmental Conditions in Bogs������������������������������������  165 10.3.2 Bog Communities��������������������������������������������������������������  166 10.3.3 Element Cycling in a Bog Ecosystem ������������������������������  167 10.3.4 Bog Ecosystems in the Landscape������������������������������������  169 10.4 Patterns and Processes in a Tidal Salt Marsh Ecosystem ��������������  169 10.4.1 Geography and Environmental Conditions ����������������������  170 10.4.2 Ecology of a Northern Salt Marsh Community����������������  170 10.4.3 Primary Production and Energy Flow ������������������������������  173 10.4.4 Nutrient Cycling in a Salt Marsh��������������������������������������  174 10.4.5 Plant-Animal Mutualistic Interactions in a Salt Marsh ����  174 10.4.6 A Final Comparison of Bog and Salt Marsh Ecosystems������������������������������������������������������������������������  175

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11 Marine Ecosystems����������������������������������������������������������������������������������  177 11.1 A Brief Overview of Relevant Policy Issues in Marine Ecology��������������������������������������������������������������������������  178 11.2 Geography of the Ocean Environment��������������������������������������������  180 11.3 Marine Communities and Food Webs��������������������������������������������  182 11.3.1 Temperate Zone Rocky Intertidal Communities ��������������  182 11.3.2 Temperate Zone Subtidal Communities����������������������������  185 11.3.3 Tropical Coral Reefs – Productive Systems at Risk from Stress and Disturbance ��������������������������������  187 11.3.4 Food Webs in the Pelagic Zone ����������������������������������������  188 11.4 Marine Primary Production������������������������������������������������������������  190 11.5 Element Cycling and Nutrient Limitation in the Oceans����������������  191 11.6 Two Case Studies That Reveal Ecological Connections in the Oceans����������������������������������������������������������������������������������  193 11.6.1 The Aleutian Archipelago��������������������������������������������������  193 11.6.2 Antarctica and the Southern Ocean ����������������������������������  194 11.7 Stress, Disturbance, and Resource Management in Marine Ecosystems ��������������������������������������������������������������������  195 12 Agroecosystems����������������������������������������������������������������������������������������  197 12.1 Background on Food Production����������������������������������������������������  197 12.2 Ecological and Environmental Implications of Modern Agriculture��������������������������������������������������������������������  199 12.2.1 Unsustainable Water Use��������������������������������������������������  199 12.2.2 Impacts of Excess Fertilizer����������������������������������������������  200 12.2.3 Herbicides and Insecticides ����������������������������������������������  200 12.2.4 Soil Degradation����������������������������������������������������������������  201 12.2.5 Loss of Plant Genetic Diversity����������������������������������������  202 12.2.6 Genetic Modification of Crop Plants��������������������������������  202 12.2.7 A Recap of Concerns��������������������������������������������������������  204 12.3 Sustainable Agriculture – An Ecological Vision for Food Production������������������������������������������������������������������������  204 12.4 Another Issue – Use of Antibiotics in Agriculture��������������������������  207 12.5 Looking into the Future������������������������������������������������������������������  207 13 Ecological Models������������������������������������������������������������������������������������  209 13.1 Modeling Goals and Objectives������������������������������������������������������  210 13.2 Considerations in the Development of a Model������������������������������  211 13.3 Steps in Building a Model��������������������������������������������������������������  211 13.4 An Example Based on a Model of a Forest Nitrogen Cycle����������  214 13.5 Applications of Biogeochemical Models����������������������������������������  219 13.5.1 TREGRO – A Model to Simulate Plant Responses to Interacting Stresses��������������������������������������������������������  219 13.5.2 TEM: A Global Model of Net Primary Productivity��������  220 13.5.3 PnET-BGC – An Integrated Biogeochemical Model��������  221

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13.5.4 An Agent-Based Model of Harmful Cyanobacterial Growth������������������������������������������������������  222 13.6 A Recap of Models as Ecological Tools ����������������������������������������  222 14 Atmospheric  Influences, Global Warming, and Climate Change ������  223 14.1 Ozone Depletion in the Stratosphere����������������������������������������������  225 14.2 Ground-Level Ozone in the Troposphere����������������������������������������  226 14.3 Acidic Deposition – An Environmental Stress for Sensitive Organisms������������������������������������������������������������������  227 14.4 Global Warming and Climate Change��������������������������������������������  231 14.4.1 The Global Climate System����������������������������������������������  231 14.4.2 Climate History ����������������������������������������������������������������  233 14.4.3 Global Warming and Climate Change������������������������������  235 14.4.4 A Time for Policy and Action��������������������������������������������  238 15 Tropical  Ecology and Deforestation ������������������������������������������������������  241 15.1 Tropical Forest Geography and Environmental Conditions ����������  241 15.2 Species Diversity in Tropical Forests����������������������������������������������  243 15.3 Tropical Forest Community and Landscape Ecology��������������������  245 15.4 The Problem of Tropical Deforestation������������������������������������������  246 15.5 Celebrating and Saving Tropical Paradise��������������������������������������  249 16 The  Challenges of Human Population Growth ������������������������������������  251 16.1 Current Conditions��������������������������������������������������������������������������  251 16.2 The Rapid Pace of Human Population Growth������������������������������  252 16.3 Implications of Rapid Population Growth��������������������������������������  253 16.4 A Brief Summary of Current Conditions����������������������������������������  254 16.5 Moving Forward and Thinking About Options������������������������������  255 Correction to: Population Ecology ����������������������������������������������������������������  C1 Epilogue������������������������������������������������������������������������������������������������������������  257 Study Questions������������������������������������������������������������������������������������������������  259 Glossary of Terms��������������������������������������������������������������������������������������������  269 References Cited����������������������������������������������������������������������������������������������  277 Index������������������������������������������������������������������������������������������������������������������  281

About the Author

Christopher  S.  Cronan  is a Professor Emeritus in the School of Biology and Ecology at the University of Maine. He earned a B.A. in Ecology at the University of Pennsylvania, a Ph.D. in Biological Sciences at Dartmouth College, and was a Charles Bullard Fellow in Forest Resources at Harvard Forest and Harvard University. He is a former director in the School of Biology and Ecology, served as interim director of the Senator George Mitchell Center, and was founding director of the Graduate Program in Ecology and Environmental Science (EES) at the University of Maine. His research with interdisciplinary teams of scientists included fieldwork in the USA, Canada, and Europe, and his teaching involved courses in ecology, biogeochemistry, general biology, plant physiology, limnology, natural resource policy, plant biology, and field ecology. He published a textbook entitled Ecosystem Biogeochemistry  – Element Cycling in the Forest Landscape with Springer Nature Publishing in 2018 and is the author or co-author of 72 peer-­ reviewed scientific articles in the journals Science, Nature, BioScience, Water Resources Research, Environmental Science and Technology, Ecological Modeling, Landscape Ecology, Tree Physiology, Biogeochemistry, Analytical Chemistry, Geochimica Cosmochimica Acta, Soil Science Society of America Journal, Environmental Management, Canadian Journal of Forest Research, Oecologia, Forest Ecology and Management, Biological Conservation, Environment, Limnology and Oceanography, Landscape and Urban Planning, Applied Geography, Plant and Soil, Journal of Environmental Quality, Water, Air, & Soil Pollution, Environmental Monitoring and Assessment, Ecological Indicators, and Holarctic Ecology.

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Chapter 1

Introduction

When my kids were young, we would hike to a secluded animal pen at the university forest where majestic woodland caribou were being raised for a wildlife experiment. Although native caribou had long since disappeared from the Maine landscape, the hope of wildlife biologists was that a herd of Canadian caribou raised in captivity could eventually be released in the north Maine woods to restore this long-lost herbivore in the regional food web. What could possibly go wrong with this well-­ intended experiment? Unfortunately, the caribou re-introduction did not end well – once the caribou were set free in the forest landscape, they were exposed to a fatal brain worm disease carried by white-tailed deer, and all of the caribou died. It was a tragic lesson in the sometimes unexpected complexities of ecological relationships and the challenges of predicting ecological outcomes. Now, many years later, this story provides a stepping off point for us to examine the science of ecology and the relevance of this discipline to our lives, critical thinking skills, and problem-­ solving abilities. Welcome to the journey! In terms of a working definition, ecology is the study of interactions and interconnections in the web of life within the biosphere. Ecology can also be viewed as a branch of science that examines relationships among and between organisms and their environment. Whatever our backgrounds or interests, there are life lessons in ecology that can speak to each of us. Although we are all destined to follow different personal career paths, each of us will exert important impacts on the ecological well-being of the Earth. Likewise, we all will draw sustenance throughout our lives from the beauty, balance, and life support system of our planet. Hence, I would argue there is ample reason for each of us to develop a working understanding of the ecological principles that influence the patterns of life on Earth. Think for a moment about the values and priorities that guide our daily lives. We are all part of a world that is dominated by technology and global commerce, where many personal and business decisions are too often governed by economic, social, and political criteria. Growing up, most of us quickly learn how to make choices within this framework; thus, our major decisions tend to reflect an emphasis on © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. S. Cronan, Ecology and Ecosystems Analysis, https://doi.org/10.1007/978-3-031-45259-8_1

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earning potential, cost savings, career advancement, time savings, leisure activities, social networks, and other priorities rooted in free-market capitalism and inherent self-interest. Unfortunately, this approach to decision making lacks a deliberate consideration of ecological and environmental values and constraints. As a consequence, many of our choices produce short-term economic gain or fulfillment at the expense of the long-term health and welfare of the biosphere and its inhabitants. Too often, our choices favor economic progress and consumer gratification at the expense of degraded air and water quality, ever expanding energy consumption, increased levels of stress, simplification of natural ecosystems, and a general deterioration of the beauty and health of the biosphere. Sadly, it seems that we have not been able to recognize and to embrace our important role as stewards and trustees of the Earth and its future generations. One of the primary goals of this textbook is to convey the perspectives and principles – and, yes, the enchantment – of ecology to a broad audience of students and life-long learners. So, how might one accomplish that task? This book is based on the belief that the science of ecology is best understood if we examine familiar ecosystems from the natural world and weave important ecological concepts into an ecosystems framework to produce integrated examples of patterns and processes in the biosphere. In the spirit of that teaching philosophy, the core of this book focuses on specific ecosystems that are familiar to most of us (e.g., forests wetlands, streams, lakes, and the like). Here, the term ecosystem refers to functional ecological units composed of organisms and their environmental surroundings. As indicated in the table of contents, this textbook begins with an overview of key life processes, basic ecological concepts, and environmental factors, and then examines a progression of core topics in population, community, and landscape ecology. Moving forward from there, subsequent chapters provide a journey across the biosphere focused on ecological dynamics in specific terrestrial, freshwater, and marine ecosystems. The final concluding chapters explore ecological models and global change issues related to air pollution, climate change, human population growth, and tropical deforestation. Taken as a whole, the chapters of this textbook are intended to provide a conceptual framework and an intellectual pathway for understanding and interpreting the ecology of the biosphere using elements of population, community, ecosystem, and landscape ecology. Equipped with this toolkit of ecological literacy, readers and students will hopefully be better prepared to make personal, business, and civic or governmental decisions that are consistent with a healthy and sustainable Earth. It is always challenging to decide how to pitch the level of a textbook to meet the needs of students. My impression is that many of the ecology textbooks on the market are aimed at providing a deep dive on the topic in an effort to prepare students for further advanced studies in ecology. And yet, I have found that students taking an introductory ecology class are often destined for other career pathways in the life or health sciences. Balancing that situation with my own passion for sharing the excitement of ecology, I have concluded that this textbook should be less about exhaustive details on each facet of ecology and more about generating a working knowledge of the science and fostering critical thinking skills that can be applied to

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real world problems. This ecology book is intended to offer that benefit to all students. Instructors will note that this book is roughly half as long as most ecology books, leaving room for the use of supplemental readings from the primary literature and novel active learning approaches. As a final note, we can all appreciate that change is inevitable – this is true in technology, in weather, in human relationships, and in the natural world that surrounds us. For the ecologist, the challenge is to understand how living systems function and to be able to predict how specific ecosystems or regional landscapes might change in response to environmental variations, disturbances, human management, and policy decisions. This book is dedicated to sharing that challenge with new students of ecology. Note:  Words printed in bold type within the text are defined in the glossary at the end of this book.

Chapter 2

Life Processes

Have you ever found yourself wondering why the saguaro cactus occurs in Arizona, but not in Virginia or why gray wolves can be found in Minnesota, but not in Alabama? A primary reason for the variations in species distributions and abundances across the landscape is that organisms differ in their abilities to function under the range of environmental and ecological conditions found at the surface of the earth. Any given habitat or biome will support and sustain only those species that are capable of performing their fundamental life processes under the prevailing environmental and ecological circumstances. To understand how species interact with each other and their environment, it is useful to begin by exploring the basic life processes that contribute to success in an individual organism. Even before an organism such as a leopard frog can engage in competition or predation, it must be able to maintain a proper balance of water, nutrients, energy, temperature, and osmotic relations. In this chapter, basic concepts of physiological ecology will be introduced as we survey selected life processes that allow organisms to maintain homeostasis, to acquire necessary resources, and to perform normal functions required for survival and reproductive fitness. In broad terms, the ecological performance of an individual organism depends on its ability to acquire necessary resources under varying conditions. In many cases, resource acquisition is neither predictable nor effortless because (i) environmental conditions are variable or changing; (ii) other organisms are competing for the same resource; (iii) predators or herbivores (in the case of plants) are preventing or interfering with resource acquisition; or (iv) resources are limiting or are affected by interactions with other factors. Although one might imagine that two neighboring plants would simply collect their solar, water, and nutrient resources as depicted in Fig. 2.1a, the reality would probably be more like the case illustrated in Fig. 2.1b. Interactions and interference often seem to be unavoidable! A major concern for every organism is the acquisition of energy resources represented by solar energy and chemical energy contained in food and reduced carbon substrates. Ultimately, most of the energy assimilated by organisms, used for © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. S. Cronan, Ecology and Ecosystems Analysis, https://doi.org/10.1007/978-3-031-45259-8_2

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Fig. 2.1 (a) Simple view of resource acquisition by two plants. (b) A more realistic view of potential interactions during resource acquisition by two plants

metabolism and growth, and passed through food webs is derived from photosynthetic plants. How does that autotrophic primary production originate and how is it influenced by biotic and abiotic factors?

2.1 Photosynthesis and Plant Primary Production The fundamental process of photosynthesis is described by the following overall reaction: CO2 + 2H2O + light energy ➔ sugar + O2 + H2O. This elegant and complex photochemical process is comprised of two primary biochemical reactions. The light reaction or Hill reaction occurs in the chloroplasts and involves absorption of light, photolysis of water, release of O2, production of ATP (an energy storage compound), and the generation of chemical reducing power in the form of NADPH. ATP and NADPH generated by the light reaction are shuttled into the Calvin Cycle to provide the energy and reducing power required to fix CO2 as a sugar molecule that can be stored, exported, or consumed in plant metabolism.

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When plant scientists measure rates of photosynthesis, they distinguish between gross photosynthesis (total carbon fixation per unit of time) and net photosynthesis (gross photosynthesis minus carbon loss in leaf respiration). Several important ecological insights are evident in the photosynthetic reaction pathway. First, this biochemical process is a sink for atmospheric carbon dioxide (i.e., it is a CO2 consuming reaction). As we shall see later in this text, this fact is very significant in relation to the dynamics of greenhouses gases and global warming concerns. A second key observation is that photosynthesis serves as a source for oxygen. In fact, life as we know it is only possible because of the atmospheric oxygen derived from the photosynthesis of green plants. A final insight is that the formation of carbohydrate products (sugars) on the right side of the photosynthetic reaction is dependent on an adequate supply of CO2, light energy, and water on the left side of the reaction. In other words, the rate and extent of photosynthesis can be limited by shortages of these and other resources in the environment. There are many intriguing and important questions we might consider regarding plant photosynthesis. For example, how does this process vary for different species and changing environmental conditions? How do variations in rates and efficiencies of photosynthesis affect the structure and function of local ecosystems? Do scientists understand enough about the broad patterns of photosynthesis to predict how plants in the biosphere will respond to rising atmospheric CO2 concentrations and climate change? Let us take a moment to address some of these questions. Light conditions are one of the most important factors controlling the rate of photosynthesis. If we follow a light gradient from beneath a shady mature forest into a sunny meadow or upward from the dark depths of the ocean, the increase in solar light intensity will be accompanied by an increase in the potential for photosynthesis. For the sun-adapted plant in Fig. 2.2, net hourly uptake of CO2 per gram of leaf increases from 0 to 60 mg as photosynthetically active radiation (PAR) rises from ~50 to 1000 energy units of μE m−2 s−1. By comparison, the shaded-adapted plant exhibits a much more limited response, with a peak CO2 uptake equal to less than 10 mg. Two important parameters can be used to compare plant responses to changing light conditions: the compensation point and the light saturation level. At the compensation point (CP), CO2 uptake in photosynthesis  =  CO2 release by plant respiration and there is no net carbon capture or growth. The plant must be exposed to light in excess of the compensation point to exhibit net gains. The CP varies among plants that are more or less shade tolerant or sun loving, and this can provide adaptive advantages or vulnerabilities. A plant with a low CP can potentially survive and persist in a shady site where light levels are too low to support a plant with a higher CP. We can also consider the ability of a plant to exploit high light levels. At the light saturation level, light intensity reaches a threshold at which all the chlorophyll reaction centers are saturated with sunlight, producing maximum photosynthesis (Fig. 2.3). As implied by the graph in Fig. 2.2, a shade adapted plant (with a low CP)

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Fig. 2.2  Increasing uptake of CO2 as a function of light intensity

Net Uptake mg CO2 g leaf-1 hr-1

8

60

Sun-adapted

40 20

Shade adapted

0

0

500

1000

PAR E

1500

m-2

2000

s-1

Fig. 2.3  Rate of CO2 uptake at the compensation point (CP) and the light saturation threshold (Pmax)

will tend to have a lower light saturation level and will not be able to exploit high light levels compared to a sun loving plant. Thus, under full sun conditions, the slower growing shade plant will be outcompeted by the faster growing sun plant. One possible complication at high light levels is that some plants may be affected by photo-inhibition, a process that occurs when high solar radiance stimulates so much photosynthetic oxygen production that it interferes with the normal function of the rubisco enzyme that facilitates CO2 fixation. Even when light is abundant, soil moisture availability can limit plant photosynthetic activity. As soil moisture declines, plants eventually reach a drought tolerance threshold beyond which water stress increases. An increase in water stress can lead to stomatal closure as a means of limiting transpiration (water loss through leaf pores). This action conserves precious water, but has important consequences for terrestrial plants  – it interrupts the uptake of carbon dioxide gas (Fig.  2.4). Unfortunately, without continuous uptake of carbon dioxide, photosynthesis declines and ultimately stops.

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Fig. 2.4  Photosynthesis declines as a function of water stress and leaf water potential

Among terrestrial plants, we can identify three contrasting adaptive strategies that reflect the influence of water stress as an evolutionary selective force shaping the structure and function of different groups of plants. Trees, shrubs, and many crops such as soybean and sunflower are classified as C-3 plants that have presumably evolved under mesic conditions where water stress is relatively uncommon and water use efficiency is not particularly high. The C-3 designation refers to the fact that these plants absorb CO2, bind it to a 5-carbon ribulose bisphosphate molecule, and then cleave the resulting 6-carbon molecule into two 3-carbon products. Although this process works well, it is not especially efficient at maximizing carbon fixation and water use efficiency. Thus, C-3 plants are relatively sensitive to drought conditions. Plants such as grasses and corn are classified as C-4 plants, and are considered to be better adapted to high light environments where water is less abundant and plants have to balance carbon gain against the threat of water stress. Plants with C-4 characteristics exhibit distinctive biochemical and anatomical features that maximize CO2 capture and water use efficiency. In the two step C-4 process, CO2 diffuses into leaf mesophyll cells and binds to phosphoenolpyruvate (PEP) in the presence of PEP carboxylase, an enzyme that has a high binding affinity with CO2. The combination of CO2 with PEP produces a 4-carbon organic acid intermediate that gives the C-4 process its name. Given the unusual binding efficiency of PEP carboxylase, a C-4 plant can potentially modulate stomatal pore opening and closing to minimize water loss, while efficiently scavenging or absorbing a large percentage of the CO2 contained in air that enters the stomatal chamber. In hot, dry conditions during daylight, one can imagine a C-4 plant opening stomata only long enough to take in fresh air, and then closing the pores while the new CO2 is absorbed by PEP carboxylase and transferred to PEP. Once the 4-carbon organic acid molecule is formed, it then diffuses from the mesophyll cell to a structure termed the bundle sheath cell. There, the 4-carbon acid is cleaved to release pyruvate and CO2. The resulting CO2

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combines with 5-carbon ribulose bisphosphate to form a 6-carbon compound that breaks into two 3-carbon products similar to those generated in a C-3 plant. Overall, the ability of C-4 plants to capture CO2 with a highly efficient enzyme and to separate the capture and fixation of CO2 in two different cellular locations – mesophyll cells and bundle sheath cells  – provides plants in this functional category with increased tolerance for water scarcity combined with the potential for high productivity in sunny warm environments. A third impressive strategy for balancing photosynthesis against the threat of water stress in arid lands is the approach exhibited by crassulacean acid metabolism or CAM plants. A CAM plant such as saguaro cactus generally cannot afford to open its stomata during the hot desert daytime, because water loss would be extreme. Instead, the cactus opens stomatal pores at night, absorbs CO2 with PEP carboxylase and PEP, and stores the resulting 4-carbon acids overnight for later use. The next day, with its stomata closed and sunlight driving the light reaction, the cactus breaks down the 4-carbon storage molecules to release CO2, which is fixed in the familiar Calvin cycle through binding of CO2 with ribulose bisphosphate and subsequent cleavage to form two 3-carbon products. Have you ever noticed foliage that seems discolored and perhaps somewhat yellowish green, rather than a more normal green hue? This visual cue of nutrient deficiency reminds us that photosynthesis can be affected by the availability of nutrients such as nitrogen, phosphorus, magnesium, iron, sulfur, copper, manganese, and chloride, which all play critical roles in the biochemistry of this process. An inadequate supply of magnesium, for example, deprives the plant of the central atom in the porphyrin ring of the chlorophyll molecule, and interferes with production of this key photosynthetic pigment. As you might imagine, a plant cannot tolerate much chlorophyll loss without experiencing a reduction in photosynthesis and growth. As much as 10–25% of the protein in plant leaves occurs in the form of ribulose bisphosphate carboxylase (rubisco), the key enzyme that catalyzes the fixation of CO2 in the Calvin cycle. Because rubisco, ATP, NADPH, and other key photosynthetic molecules require phosphorus and nitrogen, rates of photosynthesis and plant growth may be limited by the availability of these elements. When one or both of these critical elements are in short supply, plants may suffer. Similarly, increases in the supply of these elements can enhance plant productivity.

2.2 Unlocking Metabolic Energy with Cellular Respiration Respiration is the complex process by which energy captured in photosynthesis is released as heat and work in the cells of plants, animals, and microbial organisms. The prime metabolic substrates for this process include starches, sugars, fats, organic acids, and even some proteins. During respiration, energy and matter move through a stepwise series of oxidation and reduction reactions associated with glycolysis, the Krebs cycle, and the

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2.2  Unlocking Metabolic Energy with Cellular Respiration

GLYCOLYSIS

Input of substrates

Hexose-phosphates Triose-phosphates Phosphoenolpyruvic acid Pyruvic acid

Cell wall Glycerol, oils, phospholipids, amino acids, protein Aromatic phenolic compounds and lignin Ethanol, amino acids, protein

Acetyl CoA

CITRIC ACID CYCLE

Compounds derived from precursors in glycolysis and citric acid cycle

Fatty acids, cuticular wax, terpenes, isoprenoids, aromatic compounds Oxaloacetic acid Citric acid a-ketoglutaric acid Succinyl CoA Malic acid

Amino acids, nucleic acids, alkaloids Protein, chlorophyll Cytochromes

Fumaric acid

Fig. 2.5  Metabolic compounds derived from products of glycolysis and the citric acid cycle (Krebs cycle)

electron transport system. Along the way, three key events occur: (i) energy-rich ATP is formed; (ii) heat is released; and (iii) carbon skeleton intermediates are provided for a number of other essential metabolic products illustrated in Fig. 2.5. The overall reaction for aerobic respiration can be viewed as the reverse of the photosynthetic reaction:

C6 H 12 O6  6O2  6CO2  6H 2 O  Energy Hexose sugar



where the complete oxidation of one mole of hexose sugar yields 36 moles of ATP and a free energy change of −686 kilocalories per mole. It is important to note that cell respiration can occur under either aerobic or anaerobic (oxygen-free) conditions. In the case of oxidative respiration, CO2 and water are the end-products of the reaction, whereas anaerobic respiration and fermentation result in metabolic production of various reduced carbon compounds such as methane and organic acids (e.g., acetic and lactic acids). Comparing the two reaction pathways, aerobic respiration is generally much more efficient than anaerobic respiration in terms of net energy yield per mole of glucose. From a biological and ecological perspective, respiration is a key process that facilitates the cycling of nutrients, the flow of energy, and growth and maintenance of organisms and biological communities. Once solar photons and CO2 are captured

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in photosynthesis, the fixed energy and carbon must be unlocked by the process of respiration to sustain the various metabolic processes of nutrient uptake, cell construction, movement, tissue repair, synthesis of proteins and complex hydrocarbons, thermal regulation, and the like. Much like the process of photosynthesis, respiration is very strongly influenced by physical-chemical factors in the environment. For many organisms, the rate of respiration doubles for every increase of 10 °C in the temperature range between 0 to 35 °C (the so-called Q10 rule). Respiration is also controlled by the quality and quantity of metabolic substrate (i.e., food), availability of oxygen, and the metabolic activity and mass of an organism. Comparing respiration rates for different organisms, we find that a resting human consumes 10 μmoles of O2 per gram fresh weight, compared with 200 μmoles of O2 per gram for a running human, 100 μmoles of O2 per gram for a resting mouse, and 1.0 μmole of O2 per gram for a carrot root.

2.3 Energy Relations and Carbon Allocation Whether we consider a single organism or a complex ecosystem, one of the major diagnostic features of the system is its energy balance. In other words, how much energy is available, what is the energy yield or efficiency of energy utilization, and how is energy allocated or distributed in the organism or system? Since energy is stored and transferred in the form of hydrocarbon molecules in the biosphere, carbon is often used as an ecological unit of currency to quantify the storage, allocation, and circulation of energy and matter. Suppose we consider the energy budget of an individual tree – for example, a red oak tree. In any given year, the amount of energy captured through photosynthesis (energy income) will vary as a function of environmental conditions and will be variously used to meet the needs (or energy expenses) of growth, tissue maintenance, defense, storage, and reproduction (Fig. 2.6). The sugars from photosynthesis may be converted to starches and stored, metabolized into cellulose and lignin for structural growth, or used to produce ATP for energy-dependent processes such Fig. 2.6  Multiple carbon allocation choices in a plant energy budget

Income Photosynthetic Fixation of CO2 and Solar Energy

Allocation Expenses Growth and synthesis Tissue repair

Plant Metabolic Carbon & Energy Pool

Maintenance respiration Reproduction Defense against herbivores Energy storage (starch)

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as nutrient uptake or flowering. The fundamental concept here is that the initial products of photosynthesis contain energy in the form of chemical bonds and carbon skeletons that can be allocated to meet the functional and structural demands of the living organism. What happens if there is an outbreak of gypsy moths and the oak tree is defoliated? Two important problems arise related to energy relations and carbon allocation: (i) the leafless tree loses its short-term ability to capture energy and CO2 through photosynthesis, and (ii) a significant amount of the plant’s stored energy (i.e., starch reserves) may be depleted to repair or to replace the damaged or missing foliage. This may interfere with the competing demands of growth, defense, and reproduction. Eventually, if the tree is unable to allocate sufficient energy and carbon to growth, the red oak may be eliminated through competition with other trees. For long-term success, the oak tree must strike an ecological balance that maximizes energy capture and optimizes energy and carbon allocation in the face of sometimes unpredictable environmental stresses. One of the interesting dilemmas or choices encountered by trees is the allocation of biomass above and belowground to acquire light, water, and nutrient resources. As shown in Fig. 2.7, a tree must not only produce enough shoot growth to compete for light, but must also allocate energy and carbon to root biomass to access water and nutrients in the belowground environment. There is no fixed solution to this problem  – more shoot growth will tend to place limits on root growth and vice versa. A plant may achieve a reasonable balance between shoot and root allocation for normal conditions. Yet, the onset of unexpected drought may stress the tree, because it does not have a root: shoot growth allocation strategy that provides enough root biomass to exploit untapped deeper water sources during this unpredictable occurrence of water scarcity. Among the many demands drawing on plant energy and carbon resources are the costs of physical or biochemical defenses against herbivores and pathogens. For example, a plant may choose to invest in lignification of leaves in an effort to make them less palatable or another plant may generate defensive spines to ward off herbivores. Plants can also draw on an arsenal of toxins and chemical deterrents for

Fig. 2.7  Trees differentially allocate energy and growth to roots and shoots

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defensive purposes. Many plants have evolved the ability to use secondary metabolic pathways for the production of defensive chemicals that interfere with herbivory by (i) reducing the palatability of plant tissues (e.g., bitter cucurbitacins); (ii) acting as toxic deterrents against grazers (e.g., alkaloid poisons); or (iii) reducing the nutritional value of plant tissues (e.g., tannins). One intriguing example of a stealthy nitrogenous toxin is the class of cyanogenic glycosides that are produced by certain plant species. After ingestion by herbivores, cyanogenic glycosides are broken down by enzymes that release toxic hydrogen cyanide (HCN), which inhibits cellular respiration in an herbivore by interfering with cytochromes and electron transport. A second strategy observed in plant chemical defense against herbivores is the production of polyphenolic tannins that diminish the nutritional value of food. Tannins bind soluble protein in the gut of insects, thus interfering with digestion and assimilation of amino acids necessary for insect nutrition and growth. In the end, the insect may essentially starve or fail to complete development. We can also consider a consumer organism and the effects of energy constraints on its life history. One interesting case study is an over-wintering north temperate bird such as a chickadee. Winter is a season when potential food supplies are less available to this consumer, yet its energy demands are relatively high. On one hand, the chickadee needs to gather substantial food resources to maintain body heat and to produce insulating body fat. On the other hand, as the chickadee spends more time foraging for food and becomes heavier with body fat, the bird is likely to be more susceptible to predation by owls and other carnivores. Thus, the energy-based challenge for this bird during winter is to gather enough food to avoid death by freezing, while simultaneously minimizing exposure to predators. On an annual basis, the same general allocation choices depicted in Fig. 2.6 apply to animals as well as plants, and those choices are never simple or straight-forward. The interplay of ecological interactions and metabolic allocation patterns can be illustrated with a case study from the Atlantic coastal zone, where blue mussels (Mytilus edulis) are one of the bivalve species that commonly attach themselves to the rocky intertidal and subtidal substrates. Leonard et  al. (1999) were intrigued when they observed that blue mussels were more firmly attached to rock substrates in areas with lower, rather than higher tidal water velocities. They would have predicted the opposite  – that mussel attachment would be stronger in high velocity habitats to avoid being swept away by the currents. Closer inspection of the situation ultimately indicated that crab predation, which is more prevalent in lower velocity sites, might be the factor driving mussel behavior. After completing a series of field transplant experiments and controlled laboratory studies, the investigators concluded that mussels respond to increased crab predation risk with an inducible defensive system that uses energy and carbon resources to produce thicker shells and stronger networks of byssal threads for attachment to the rocky substrate  – both of which provide protection against crab predation. We can turn to the polar bear, an arctic apex predator, for a final example of the importance of energy relations. We all may be familiar with news reports of threats from global warming for polar bears, but can we account for this situation in ecological terms? According to a research report by Pagano et al. in 2018, the threat of climate change for polar bears (Ursus maritimus) centers on a potential imbalance

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in energy supply and demand. Polar bears have an active, high-energy, and high-fat lifestyle that is at risk, because a decline in arctic sea ice is reducing access to and abundance of their preferred seal prey. The root of the problem is that the polar bear has an elevated resting metabolic rate or basal metabolism, as well as an elevated field metabolic rate that includes movement and hunting; as such, energy demand increases with polar bear size and foraging activity. To remain in energy balance with their elevated metabolic demands, polar bears have evolved hunting tactics to prey on energy-rich ringed seals and similar marine mammals with rich blubber. An adult polar bear needs to consume an adult ringed seal every 10–12 days simply to maintain energy balance; moreover, body growth and feeding of young can increase this energy requirement. Over the course of three-years, Pagano et al. observed that more than half of the bears in their study lost body mass, which means that their energy demand exceeded that gained by consuming prey. Future increases in activity and movement resulting from declining and fragmented sea ice are likely to increase the demand side of the energy balance ratio, posing risks for body condition, reproductive success, population sizes, and survival of polar bears.

2.4 Water Relations Water is fundamentally important for the normal functioning of virtually all organisms and ecosystems. Most physiological processes occur in the presence of water, and water is the medium through which organisms typically acquire their essential nutrients. As a consequence, variations in the availability, quality, and circulation of water have a major influence on the abundances, distribution, and health of life forms in the biosphere. To introduce the basic concepts of water relations in living organisms, we can focus on examples from the plant and animal kingdoms. For simple illustration, let us consider the water relations of a flowering Rhododendron, an ericaceous perennial woody shrub often found growing in the sun-flecked understory of temperate deciduous forests in North Carolina and elsewhere. Water absorbed through fine roots is transported upward in the xylem, absorbed along the way by various cells and tissues, and is eventually released to the atmosphere as water vapor via the process of transpiration. Most of the transpired water exits from the leaves through microscopic pores known as stomata. The driving force for this uptake and movement of water is a gradient based on differences in water potential along a pathway from the soil, through the plant, and into the atmosphere. A water potential gradient develops in response to the plant water deficit created by water loss to the dry atmosphere and from the osmotic potential associated with ion and solute accumulation in plant cells. The basic concepts of water potential are summarized in Table  2.1, and the overall process of water movement from soil to plant to the atmosphere can be described as follows. In the soil environment, the water potential is somewhat negative as a result of dissolved solutes in soil water and because of the tension (or negative pressure) created by the attraction of water molecules to surfaces of soil particles. At the wilting

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Table 2.1  Water potential concepts Water potential is (a) a physical property that explains water movement in plants, other organisms, and soils; (b) a measure of the free energy content of water; and (c) is abbreviated by symbol psi, ψw. Water potential is expressed in units of pressure called megapascals (MPa), where 1 MPa = 10 atm, home water pressure is about 0.25 Mpa, and soil field capacity is – 0.03 MPa. Pure water has a ψw = 0 and the addition of solutes lowers ψw to less than 0. For example, a solution with 0.1 M solute concentration has a water potential of −0.24 MPa. We can expect the following relationships: Increasing the solute concentration lowers ψw, increasing the pressure raises ψw, and negative pressure (i.e., tension) lowers ψw. Water moves from a zone of higher water potential to a zone of lower water potential. For example, water moves from a zone where ψw = 0 to a zone where ψw = −0.5

point, the tension is −1.5 MPa, whereas at field capacity, tension is −0.03 MPa. As roots withdraw water from their surroundings, a micro-zone of soil water depletion is created adjacent to the root. Water potential in the micro-zone decreases as the remaining water near the root is held with greater tension by soil particles. That slight depression in water potential around the root creates a gradient that allows soil water in the surrounding area to diffuse toward the root. For water to move from the soil to the plant, water molecules must follow a gradient from a higher water potential in the soil to a lower water potential in the plant. Remember that this is all relative, so that water may move from a soil zone with a water potential of −0.1 to the interior of a plant with a water potential of perhaps −1.0 MPa. How does the plant develop a lower water potential than the soil? There are two components to this: solute or osmotic potential and pressure potential. As plant cells accumulate sugars or other solutes, the osmotic potential decreases (becomes more negative) and this makes the water potential more negative inside the plant. Even more important than that process is the development of negative pressure or tension in the plant from the surface tension of water at liquid-air interfaces inside plant cells. The details of this phenomenon can be explored in a plant physiology class, but suffice it to say that this tension works somewhat like using a straw to suck up liquid. If we shift to the air surrounding plant leaves on a sunny warm day, the air will have a vapor pressure deficit, which corresponds to a low or negative water potential in the air. This is essentially the force that “sucks on the straw.” Plant moisture inside the leaves responds to the low water potential in the air by evaporating and this reduces the moisture content in the leaf. As the moisture films in the mesophyll lose water, the remaining water contracts against surfaces inside the plant and there is an increase in surface tension of the water, which translates into negative pressure and a decrease in the leaf water potential. When this water potential drops below the water potential in the soil (say −1.5 MPa in the plant versus −0.1 MPa in the soil), a gradient develops and water molecules follow that water potential gradient from the soil, through the plant, into the leaves, out the leaf stomata, and into the warm dry atmosphere.

2.5 Osmoregulation

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In the final analysis, when there is a water potential difference between the plant and the soil, moisture in the soil responds to the water potential gradient like a ball rolling down a hill in response to gravity. The process of plant water uptake will continue as long as there is a sufficient water potential gradient to extract water from the soil. If the soil becomes too dry or freezes, or if transpiration ceases, water movement into the plant will decrease or stop. Inside the Rhododendron plant, water is used to maintain turgor pressure in the leaves and tissues, as a liquid medium for transport of sugars, ions, and other solutes, and as a solvent for various biochemical reactions in the plant. Water also provides the driving force for cell expansion associated with tissue growth in the plant. Because it is such a precious resource, terrestrial plants have evolved mechanisms for conserving water. In the case of the Rhododendron, one of the most obvious adaptations for water conservation is the morphology of the leaf, which is covered with a thick waxy cuticle to minimize water loss and to prevent dehydration. Since the cuticle of terrestrial plants is relatively impermeable to CO2 and O2, plants have evolved stomata to permit gas exchange. Yet, stomata also allow water vapor loss during CO2 uptake. In the final analysis, plants must balance transpirational water loss and gas exchange to maintain photosynthesis and to enable the flow of water required for nutrient circulation, plant growth, and leaf cooling. As an example from the animal kingdom, we can examine water relations in the kangaroo rat, a creature that faces a serious dilemma in achieving water balance in the arid desert environment. Here, we have an animal that lives where water is extremely scarce and where most available water evaporates quickly into the hot, dry desert air. In the face of this environmental challenge, the rat has evolved the ability to survive on water obtained directly from the food it consumes, from metabolic water released during cell respiration, and from the small amounts of dew and precipitation that reach the ground surface. The rat conserves water with its very efficient kidneys and intestines that resorb and recycle internal water, thus allowing only minimal excretion of moisture in its concentrated salty urine and dry feces. The kangaroo rat also minimizes evaporative water loss by means of nocturnal behavior, avoidance of prolonged direct exposure to the hot sun, and a physiological adaptation that allows recovery of water vapor from exhaled air leaving the lungs. As moist air exits through the rat’s enlarged nasal passages, the moisture condenses on the nasal membranes, preventing its loss to the desert air.

2.5 Osmoregulation The physiological processes of osmoregulation allow an organism to maintain homeostasis in conditions of elevated salinity, osmotic stress, drought, or freezing temperatures that may threaten the survival of a microbe, plant, or animal. Picture an anadromous fish such as the Atlantic salmon (Salmo salar) that hatches in a

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freshwater river, spends much of its life in the salty ocean, and then migrates to freshwater spawning grounds to reproduce. Imagine the osmoregulatory challenge for this fish! In freshwater, a fish is surrounding by water that is dilute compared to the osmotic potential of body fluids. As such, the fish is considered to be hyperosmotic relative to its external medium, and this means that the fish tends to absorb too much water and to lose salts easily through diffusion. Under these conditions, a fish can maintain osmoregulation by seldom drinking, by excreting copious dilute urine, and by actively absorbing salts through the gills. In a marine environment, a fish is bathed in ocean water with an elevated salinity of roughly 3.5%, which means that the fish is hypoosmotic compared to the external medium. Under these circumstances, the fish tends to lose internal water to its surroundings and to absorb excess salt through passive diffusion. It can compensate for this imbalance by drinking seawater continually and excreting salts across its gill membranes.

2.6 Gas Exchange Processes of gas exchange are fundamentally important for living organisms, especially the exchanges of oxygen and carbon dioxide. If you have ever taken a deep free dive to get closer to a benthic coral reef, you may recall the stress of holding your breath way too long, when you really wanted to gulp a new lungful of fresh oxygen. This diving scenario reminds us that when gas exchange is interrupted in any organism, basic life processes can suffer. Plant photosynthesis and cell respiration present an interesting dichotomy with respect to the gases CO2 and O2. Whereas gas exchange in a plant leaf is dominated by CO2 uptake and O2 release during daytime, the opposite is true for a heterotrophic mammal – it absorbs O2 and releases waste CO2. Vascular terrestrial plants have evolved the ability to control leaf gas exchange with specialized guard cells that open to promote CO2 uptake and close to minimize loss of water vapor. When the guard cells detect sufficient light and internal moisture, membrane-bound proton pumps initiate an influx of K+ ions that lowers the osmotic potential of the cells and stimulates an influx of water responding to the high concentration of potassium ions. As the guard cells absorb more water, their turgor pressure increases, the guard cells change shape, and the stomatal pore opens to permit gas exchange. Later, when light or moisture may become limiting, the process can be reversed by pumping K+ ions out of the guard cells, lowering the osmotic potential, decreasing the turgor pressure, and generating stomatal closure. Animals with hemoglobin have their own amazing adaptation for efficient gas exchange. In the lungs, where oxygen concentration is high and blood pH is somewhat elevated, iron-rich hemoglobin binds tightly to O2 molecules. When oxygenated red blood cells circulate to other parts of the body where active respiration is occurring, the hemoglobin responds to the conditions of low pO2 and elevated pCO2

2.7  Heat Balance and Thermal Regulation

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(which lowers the pH slightly) by releasing oxygen and absorbing some of the waste carbon dioxide for transport out of the body. In this way, hemoglobin acts as a transfer molecule that is essential for gas exchange.

2.7 Heat Balance and Thermal Regulation Thermal conditions, heat tolerance, and cold tolerance are major factors determining the range limits, geographic distributions, and performance of many organisms in the biosphere. Animals exposed to extremes of hot or cold temperatures must have the ability to avoid lethal heating or cooling of body tissues through behavioral or physiological mechanisms of thermal regulation. Solutions include body insulation, metabolic regulation of internal temperature, evaporation, hibernation, or avoidance. As a simple but elegant evolutionary example of an adaptation for thermal stress, consider a seagull standing on ice in mid-winter. How does the bird maintain its core temperature and avoid freezing? Part of the answer is a circulatory system that allows for counter-current heat exchange. Outgoing arterial blood headed toward the lower legs and feet transfers most of its heat to returning venous blood, so that the blood entering the feet is cool and has much less heat to lose to the frozen ice. As venous blood moves upward again from the cool feet, it is reheated by counter-current heat exchange with adjacent warm arteries, so that the blood returning to the heart has a normal body temperature. The heat load on a plant leaf in full sun is very high and can potentially push the leaf temperature above its physiological tolerance. How does a plant manage its energy budget and heat balance during the summer growing season so as to maintain thermal homeostasis? The answer can be understood in the context of a heat budget as illustrated below: H s  H m  H r  H cd  H cv  H e where Hs  =  heat stored, Hm  =  metabolic heat, Hr  =  radiation, Hcd  =  conduction, Hcv = convection, and He = evaporation. For most plants, one of the major determinants of the heat budget is the radiation term, Hr. Plants can influence how much solar radiation is absorbed with two major mechanisms: changes in leaf orientation (toward or away from direct sunlight) and changes in albedo or reflectivity of the foliage (which may be influenced by cuticular wax, leaf hairs, and spines). Plants can also re-radiate long wave IR radiation to dissipate some of the solar gain. The next term in the heat budget, Hcd, indicates that a plant may gain or lose heat by contact with an adjacent plant or soil through the process of conduction. When there is a temperature gradient between the plant and the surrounding air, the plant can lose or gain heat through the physical process of convection, Hcv, which can be a particularly effective way to dissipate plant heat load on a breezy day. The final essential component in the heat budget is the term He, which represents heat loss through evaporative cooling. The transformation of

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liquid water to vapor requires a large input of heat (540 cal g−1), which is why plant evapotranspiration is such a powerful way to remove heat from a plant leaf. Every gram of water vapor released through the guard cells of a plant leaf transfers a substantial amount of latent heat to the surrounding atmosphere. Consequently, this is a major mechanism for achieving thermal regulation in a plant. A final option for plants is to drop leaves in extreme heat, so as to avoid lethal heating or unsustainable water loss. Saharan silver ants (Cataglyphis bombycina) that forage under extreme thermal conditions in the African desert provide an illustration of a morphological adaptation that allows modulation of radiation balance in its heat budget. These silver ants are covered with a dense array of triangular hairs that provide two benefits for desert survival: the hairs enhance albedo or reflectivity of solar radiation, which lowers heat gain, and the hairs also increase the emissivity of the ant, which enables the animals to dissipate heat back to the surroundings via blackbody radiation (Nan shi et al. 2015). On the opposite side of the thermal ledger, we can consider freezing tolerance. How do perennial plants in cold climates survive subfreezing temperatures? The needles of a high elevation spruce tree prepare for winter through a metabolic process of cold hardening that generates natural organic solutes that act as a biological antifreeze. Cued by decreasing day lengths and cumulative exposure to increasingly cold temperatures, the plant produces sugars and alcohols that lower the freezing point of the cell sap and protect the foliage from the damaging effects of ice crystal formation. As shown in Fig. 2.8, the step-wise process of cold hardening unfolds from fall through the beginning of winter and then when spring arrives, the process can be reversed. Survival of foliage in a given year and location may depend on the rate at which cold hardening proceeds, the final level of cold tolerance achieved and maintained by the plant, and the incidence of temperatures below the level of cold tolerance.

Fig. 2.8  Temperate and boreal woody plants undergo cold hardening prior to winter, with increasing tolerance to subzero temperatures shown in oC

2.8  Nutrient Relations

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2.8 Nutrient Relations Nutrient elements are the building blocks for a wide range of structural and functional molecules that permit the growth, survival, and reproduction of the diverse life forms that inhabit the biosphere. Our ability to understand the biology of organisms and the ecology of natural ecosystems requires a working knowledge of the major nutrients, their biological roles, and their linkages with water resources, energy, and a diversity of life processes. The elements summarized in Table  2.2 are probably familiar by name, but to what extent can we identify critical roles associated with each one? One remarkable example is the element phosphorus, a nutrient that makes life possible through multiple structural, genetic, and energetic functions. Phosphorus is an essential component of skeletal material (as calcium phosphate) and membranes (as phospholipids), an indispensable part of DNA and RNA, and a vital part of the energy transfer molecule, ATP. In the case of calcium, we have an element that (i) plays a major structural role in skeletal material, plant cell walls (as calcium pectate), and shells of aquatic invertebrates; (ii) contributes to ion selectivity in cell membranes; (iii) facilitates action potentials in the nervous system; and (iv) acts as a signaling compound in a variety of organisms. Two final examples from Table 2.2 are the elements potassium and sodium, which primarily serve as electrolytes in the cytoplasm and membranes of cells. These soluble ions contribute to osmotic homeostasis in cells and help to control membrane chemical potential gradients. Along with these four examples, take note of the important functions of nitrogen, sulfur, magnesium, and iron in the table. Before leaving this short introduction to nutrient elements, let us follow the pathway of a nutrient from the soil into a living plant as an example of the process of nutrient acquisition. One form of nitrogen that is readily available to plants is the nitrate ion, NO3−. Nitrate in the soil solution bathing the root system must move by diffusion into the so-called “free space” of the root cell wall. When the NO3− ion reaches the plasmalemma membrane of a root cell, the negatively charged ion is taken across the membrane by an ATP-dependent active transport system fueled by energy derived from root respiration. The nitrate nitrogen may then be used in the root system or transported through the xylem stream to another part of the plant. To Table 2.2  Important plant nutrients

Nutrient Nitrogen (N) Phosphorus (P) Calcium (Ca) Magnesium (Mg) Sulfur (S) Potassium (K) Sodium (Na) Iron (Fe)

Examples of Roles or Functions Proteins, enzymes, DNA, chlorophyll DNA, ATP, phospholipids, skeleton Structure, membranes, signaling Enzymes, chlorophyll Amino acids, co-enzymes Electrolyte Electrolyte Enzymes, ferredoxin, hemoglobin

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be incorporated into protein synthesis or plant metabolism, the nitrate ion must be transformed by the enzyme nitrate reductase into ammonium ion (NH4+) at the expense of energy derived from plant respiration. After that investment of energy to reduce nitrate to ammonium, the NH4+ can be used in the synthesis of amino acids, nucleic acids, and other metabolites.

Chapter 3

Environmental Analysis

Imagine you have a job with Trout Unlimited and your boss asks you to set up a sampling program to determine which environmental factors are the best predictors of streams capable of supporting brook trout populations. Think about which environmental factors would be at the top of your list of primary determinants of ideal brook trout habitat. Perhaps you would include summer maximum temperature, minimum concentration of dissolved oxygen, pH, mean concentration of dissolved calcium, vegetation conditions along the stream channel, amount of suspended sediment, and food resources on your list of prospective factors. Maybe you would seek out the wisdom of the Maine Guides Association to learn where they look for ideal trout habitat. Ultimately, you would probably have to assemble a massive data base of habitat parameters and would then explore statistical relationships among the many biophysical and chemical attributes associated with streams that either support or lack brook trout populations. Whether we are interested in brook trout or some other group of organisms, it is important to recognize that the distributions of different organisms can be influenced by variations and combinations of a wide range of environmental parameters. As such, one of our fundamental challenges in ecology is to figure out how to analyze and to interpret environmental variations in the biosphere. In this chapter, we will be guided by four primary questions concerning environmental conditions that affect organisms: (i) Which environmental factors are significant? (ii) How do environmental factors vary on earth? (iii) How can environmental conditions be described or measured in a meaningful way? (iv) What are the implications of environmental changes for species, communities, and ecosystems?

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. S. Cronan, Ecology and Ecosystems Analysis, https://doi.org/10.1007/978-3-031-45259-8_3

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3.1 Environmental Gradients We know that environmental conditions are non-uniform in the landscape, and that is why, for example, birds fly to warmer regions in winter. Environmental heterogeneity is the result of variations in multiple climatic, geologic, topographic, geographic, and biological factors. Amidst this mosaic fabric of variables, one of the common patterns observed across the biosphere is the presence of environmental gradients characterized by a continuum (i.e., a vector) of changing conditions. As one example, we can identify a global thermal gradient that extends from frigid polar latitudes, through the temperate zone, and into the hot equatorial zone. Along this environmental gradient, the changing thermal conditions provide a basis for separation of organisms that are more narrowly or broadly adapted to a given temperature regime. One of the locations where striking environmental gradients occur is in mountainous terrain. If you have ever climbed a mountain or viewed a snow-capped peak from a distance, you may have a sense of how environmental conditions change from the valley floor upward toward the mountain summit. Generally, we expect temperature to decrease with elevation, while precipitation, cloud cover, and snowfall depth all increase. How do we account for these variations? An explanation for these patterns begins with the concept of adiabatic cooling, which is governed by density and pressure relationships in air masses. At a low elevation, air pressure and density are higher and this favors increased molecular interactions, kinetic energy, and heat. At a higher elevation on a mountainside, air pressure is lower, air expands, kinetic energy and heat content decrease, and the air becomes cooler. This concept is illustrated with the balloon diagram in Fig.  3.1, which shows that air temperature on a mountain summit at 2000 m could be more than 10  °C cooler than the temperature measured at the base of the mountain at 500 m elevation. The cooling that originates from adiabatic processes creates the conditions that allow for more rainfall, snowfall, and cloud cover. As air masses move upward over a mountain, the air cools until it reaches the dew point at 100% relative humidity; Fig. 3.1 Conceptual diagram of adiabatic cooling as an explanation for montane temperature gradients. Above the dew point elevation, air cools at a slower wet adiabatic rate (thus, the change in slope)

3.1  Environmental Gradients

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this allows for water vapor condensation, cloud formation, and the onset of orographic precipitation. In winter, this temperature-moisture relationship at higher elevation can increase the frequency and duration of snow events. Downwind of a mountain range, a rain shadow may occur, because air passing over the high elevation mountains loses moisture to orographic precipitation. Consequently, there is little cloud water vapor left to produce precipitation in the rain shadow area. An example is the arid Mojave Desert located to the east of the Sierra Nevada Mountains in California.

3.1.1 An Example of a Montane Gradient in the Southwestern U.S. Outside the city of Tucson, Arizona, the Santa Catalina Mountains rise from the Sonoran Desert at 700 m to a summit elevation of 2800 m. What would be your predictions for the ecological and environmental changes we might observe along an elevation gradient in the Santa Catalina Mountains? Do we expect arid or semi-­ arid conditions to prevail from the valley floor to the mountain summit? Early studies by Robert Whittaker and Bill Niering provide ecological clues that can help us to discern and to understand how conditions change with elevation in the Santa Catalinas (Whittaker and Niering 1968; Niering and Lowe 1984). Based on their field observations, there is a pronounced vertical zonation of plants and ecological communities as you move upward in elevation on the Santa Catalina Mountains. Hiking along the gradient, we transition from desert habitat, through desert grasslands, into woodland habitats with oak and pine-oak vegetation, and then climb through successive zones of pine forest, fir forest, and spruce forest at the summit. These striking patterns of changing vegetation imply that there are significant underlying differences in environmental conditions along that elevational gradient. Two of the major variables that change along the gradient are temperature and precipitation – mean annual temperature decreases by 7.5 °C with every 1000 m of elevation, whereas precipitation increases by 15 cm per 1000 m of increased elevation. The warm and arid conditions at the base of the mountain limit the survival of woody vegetation and contribute to increased fire frequency, which favors grasses over perennial forest vegetation. Another factor that limits vegetation is grazing pressure from cattle that forage on plants at low elevations. Moving upwards in elevation, the increase in precipitation is accompanied by increased soil moisture that favors plant production and the accumulation of organic matter in soils. There is also a decrease in frost-free season in the upper elevations, which favors hardy coniferous trees over plant species adapted to conditions at lower elevations. One final noteworthy feature of the montane gradient in the Santa Catalinas is the fact that the vegetation zones occur at different elevations, depending on slope aspect (i.e., the compass bearing of a location on the flank of the mountain). If one circles

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around the perimeter of the Catalinas, it becomes apparent that zones are pushed upwards on southern and southwestern aspects. The explanation for this pattern is that S and SW aspects represent hotter and drier microclimates than N and NE exposures, which means that a forest community that occurs at 2000 m on a NE aspect must move to 2500 m on a S/SW exposure to obtain enough precipitation moisture to support the forest vegetation.

3.2 Geographic Ranges of Species in Relation to Environmental Factors The geographic ranges and distributions of species are heterogeneous and provide an intriguing puzzle that challenges us to identify which environmental parameters are important in determining differences in the biogeography of different species and taxa of organisms. Take an example of the two temperate forest species – red maple and red spruce – shown in Fig. 3.2. The range maps for these trees overlap in some locations where the species may co-exist, but otherwise, the maps differ substantially. Are there environmental conditions that help to account for these distribution patterns? In general, we see that red maple is distributed broadly across the eastern U.S. from Florida in the south to New England and eastern Canada in the north. By comparison, red spruce occurs in the northeastern U.S. and eastern Canadian provinces, and is largely absent south of Massachusetts and New York, except for higher elevations in the Appalachian Mountains extending as far south as the Smoky Mountains. The evidence suggests that red maple is a generalist that can tolerate a wide range of temperatures, but that it cannot extend further west than the Mississippi River Valley. Given that annual rainfall decreases from east to west, we might

Fig. 3.2  Geographic range maps for red maple (left) and red spruce (right). (From USDA 1965)

3.3  Patterns of Light and Solar Radiation

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hypothesize that diminished moisture availability limits red maple as it tries to move westward into the Midwestern U.S.  In the case of red spruce, it appears to be a northern and high elevation species that either cannot tolerate warmer summer temperatures or cannot compete with tree species that are better adapted to warm or hot conditions. Other factors may also be at play here. What kinds of thermal measurements are meaningful for describing species distributions in relation to temperature? One approach is to focus on degree-days, a metric that is computed as the sum of daily temperatures above freezing (0 °C) for the year. As an example, the geographic range boundaries of yellow birch (Betula allegheniensis) within North America correspond roughly with 2000 degree-days per year in the north and 5300 degree-days in the south. This implies that yellow birch cannot tolerate northern locations where the annual heat input is insufficient to equal or to exceed 2000 degree-days. Other potentially useful thermal metrics include mean annual temperature, mean summer temperature, and mean winter temperature. If we consider the northern range limit for species, it may be extreme winter cold periods that determine whether a species can survive and reproduce; thus, a map of mean minimum winter temperatures may be informative in our efforts to understand why some species are restricted in the northward extension of their range. For example, a species would have to tolerate minimum winter temperatures in the range of 5 °F to survive in northern Georgia, whereas a plant in northern Maine would encounter much more severe mean minimum winter temperatures in the range of −25 °F.

3.3 Patterns of Light and Solar Radiation There is no doubt that the sun makes a huge difference for our planet – without it, Earth would be a dark frozen orb in the universe. Thanks to inputs of solar radiation, plants can grow and produce food and oxygen, and life as we know it is possible. Solar energy enters the outer atmosphere of the Earth at a rate of 20 kilocalories m−2 min−1, a value that is referred to as the solar constant. Of the total radiation entering the upper stratosphere, about 35% is reflected back into space (by dust, clouds, sulfate aerosol particles, and the earth’s surface), 15% is absorbed by clouds and gases, and 50% is absorbed by land and ocean surfaces. The actual amount of solar energy available to organisms at the surface of the Earth varies considerably as a function of latitude, season, elevation, topography, cloud cover, and microclimate. For example, the annual solar energy input at the North Pole (roughly 125 kcal cm−2 year−1) is less than half of the solar input at the equator (>300 kcal cm−2 year−1). At a latitude of 43oN, the length of the daily photoperiod varies by as much as 6 h between winter (~9 h) and summer (~15 h). This seasonal variation of the light regime influences the timing of life cycle events and the duration of the growing season for plants. A map of cloud cover frequency in the continental U.S. indicates that the average number of cloudy days varies from under 40 to 160 or more across the entire country. Because clouds can reflect substantial

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amounts of incoming solar radiation, an increase in cloudiness can translate into potentially significant reductions in the amount of light and heat available to the local environment. In hilly terrain, smaller scale differences in microclimate associated with topographic relief and slope aspect can affect solar inputs to the ground surface. For example, a south-facing slope may receive twice the input of solar energy in winter as a north-facing slope. This topographic influence on solar insolation can affect snowmelt patterns, watershed hydrology, and the occurrence of winter desiccation and injury in terrestrial plants. On south-facing slopes, snow typically melts earlier and faster, and plants are more likely to suffer stress or injury from excess solar heating or rapid temperature fluctuations during winter dormancy. At the ecosystem level, important microclimatic variations in light can be observed along horizontal and vertical spatial gradients. As an example, the top of a forest canopy or the surface of a lake routinely receives full sunlight that promotes vigorous photosynthesis. However, beneath a dense forest canopy or below the photic or well-lit zone in a lake, light can diminish to a level of 5% full sunlight or less. The quality of the remaining light also changes as portions of the wavelength spectrum are absorbed in passage through the plant foliage or water column. In horizontal space, light conditions can vary from sunny gaps to shaded zones as a result of structural heterogeneity in the plant community and time of day. Any and all of these variations in the quantity and quality of light can modify environmental conditions in ways that influence patterns of plant growth and ecosystem energy flow.

3.4 Climatic Patterns – Temperature and Moisture Global variations in solar energy input directly affect the thermal environment across the surface of the Earth. When we wonder why a palm tree grows in Florida and not in Norway or why a giant brown kelp flourishes in the cold waters of the Gulf of Maine, but not off the coast of Puerto Rico, we find that an important part of the answer involves different tolerances of these organisms to patterns and extremes of thermal conditions. There are broad geographic variations in annual and seasonal temperatures, and this mosaic pattern of thermal heterogeneity acts as a constraint that influences distributions of species. If we examine temperature data for North America as an example, mean January temperature along the east coast ranges dramatically from 10 °F in Maine to 65 °F in Florida, whereas mean July temperatures span a smaller range from 65 °F in Maine to 80 °F in Florida. Imagine palm trees in Miami trying to move north to colonize the Maine landscape with its harsh winters! Uneven heating of our planet by solar radiation also produces striking variations in ocean temperature, ranging from polar waters at ≤0 °C to the Pacific warm pool off New Guinea, where water temperature averages 29.5  °C.  These marine

3.4  Climatic Patterns – Temperature and Moisture

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temperature differences and associated changes in water density help to drive a global network of warm and cool ocean currents that interact strongly with global climate and provide long-range connections and heat transfer among different parts of the Earth. As one example, weather across North America is sometimes influenced by interactions between the warm water pool of the western Pacific and the El Niño current of the eastern Pacific, which contribute to a cyclic pattern of abnormal rainfall and temperature controlled by the El Niño Southern Oscillation. Moisture availability is another key environmental parameter that can be characterized in a variety of ways that may be meaningful for interpreting the distributions of species. At a continent spatial scale, mean annual precipitation varies by a factor of 10 across the United States, decreases by 50% from the east coast to the interior Midwest, and ranges widely from the Mediterranean climate of southern California to the temperate rain forests of the Pacific Northwest. Furthermore, many locations exhibit striking inter-annual variations around the mean, with wet years receiving twice as much precipitation as dry years. Another critical aspect of precipitation patterns is seasonality. Precipitation may be more or less evenly distributed over the yearly calendar or moisture inputs may be skewed toward a wet summer/dry winter or dry summer/wet winter type of climate. Likewise, different locations in the temperate zone may have either a minimal or a large proportion of precipitation in the form of snow and this can be a difference-maker. Parts of the western U.S. depend heavily upon winter snowpack meltwater as a source of recharge for groundwater, rivers, and reservoirs, because summer rainfall is so limited. Ultimately, the availability of water and the nature of precipitation patterns have a powerful influence on species distributions. Depending on moisture conditions, a given environment may support what are termed hydric, mesic, or xeric habitats. At the xeric end of the precipitation spectrum, we find desert ecosystems that generally experience rainfall amounts of 0–25 cm (10 inches) per year. At the other extreme, annual rainfall in temperate and tropical rainforest regions of the Pacific Northwest, Amazon Basin, west-central Africa, and southeastern Asia ranges from 200 to as high as 500  cm  year−1. In between the xeric and hydric ends of the moisture spectrum, mesic humid temperate regions typically receive roughly 40–100 cm of annual precipitation as combined rainfall and snowfall. A question we might ask regarding rainfall patterns is how can we account for the geographic locations of deserts versus rain forest habitats? The majority of rain forests with high rainfall are located along the equatorial region where the tremendous solar input drives warm moist air into the upper atmosphere, leading to moisture condensation and persistent rainfall. One interesting exception to this equatorial pattern of high rainfall is the U.S. Pacific Northwest, which is downwind of westerly onshore breezes from the northern Pacific Ocean. When moist Pacific air masses reach the cool land mass and condense, the resulting precipitation is sufficient to support a lush temperate rainforest environment in Oregon and Washington. Whereas some deserts occur in the rain shadow downwind of a mountain range, the large continental deserts that occur in Africa, Asia, and other

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parts of the globe are largely the result of macroscopic global circulation patterns. There are six major atmospheric circulation cells that span zones of roughly 30° latitude proceeding away from the equator. At the equator, heated air masses rise, cool, drop their moisture, and flow poleward toward the subtropics. These cool, dry air masses eventually descend and flow back toward the equator as trade winds. Many major continental deserts coincide with locations around 30° north and south latitude, where the descending dry air masses experience adiabatic warming, causing these ever warmer and drier air parcels to scavenge any available water from the landscape. As a result, the land becomes parched and is rarely replenished with rainfall. By integrating what is known about atmospheric circulation patterns, marine currents, and temperature gradients across the globe, we can begin to make predictions about the climatic conditions expected in different parts of the world. Thus, the temperate rainforests in western Washington, the dry fire-prone Mediterranean climate in southern California, the broad expanses of desert in northern Africa, and the humid temperate conditions of the British Isles can be accounted for by the interplay of atmospheric and oceanic processes affecting heat and moisture. In closing this section, we should note that despite many ecological complexities surrounding climatic factors, it is possible to discern relatively simple and straight-­ forward relationships for some parameters. As an illustration, the graph in Fig. 3.3 combines global climatic data to illustrate how just two variables – mean annual temperature (MAT) and mean annual precipitation (MAP)  – provide a basis for separating major biomes into different climatic zones in the biosphere. This graph implies that although there are many finer scale influences on the biogeography of organisms, broad patterns of species distributions can potentially be explained by relatively simple combinations of environmental factors. Fig. 3.3 Climatic conditions associated with different biomes (tropical rain forest A, tropical seasonal rain forest B, savanna and thorn scrub C, desert D, temperate rain forest E, temperate forest F, grassland G, taiga H, and tundra I). (Data from Ricklefs 1983)

3.5  A Case Study Exploring the Relationship of Plant Diversity and Environmental…

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3.5 A Case Study Exploring the Relationship of Plant Diversity and Environmental Influences Data collected by a graduate student in Maine provide an opportunity for us to examine the potential influence of environmental factors on a regional pattern of plant diversity. Using these data, we can ask how do environmental conditions and species diversity patterns vary in Maine and are the two related? A null hypothesis for our investigation could be stated as follows: the Maine landscape is homogeneous and exhibits little environmental variation; therefore, spatial patterns of woody plant species diversity are independent of environmental conditions. So, how do the data match up? McMahon (1990) reported that there is a two-fold range of variation in the number of woody plant species in the state, with highest species richness along the southern coast (190–199 species) and lowest richness in the northwestern forest landscape (90–99 species). A second interesting pattern that was observed is that a large number of species either reach their southern or northern range boundaries in Maine. In other words, these are either northern species that are unable to move further south or southern species that cannot move any further north. To link these observations with environmental patterns, we should probably begin with the null hypothesis and determine whether there is appreciable environmental variation that might account for or correlate with the heterogeneity of plant species diversity. Scientists have delineated 15 biophysical regions in Maine based on topography, climate, and other parameters. This is our first evidence that the region represents a heterogeneous environment suitable for many different species. More detailed patterns of relevant environmental parameters were examined by McMahon (1990), indicating that (i) mean annual rainfall is 50% higher on the south-central coast (where diversity is higher) than in the northwestern zone (where diversity is much lower); (ii) mean annual snowfall is over 50% higher in north-­ central and northern Maine as compared with southern Maine; (iii) mean minimum temperatures in winter range from −20 °C in southern Maine to −32 °C in northern Maine; and (iv) the frost-free growing season lasts half as long in northern as compared with southern Maine. Overall, we have an initial body of evidence that suggests, but does not prove, that the lower incidence of woody plant diversity in N/NW Maine may be related to any or all of the following environmental factors: decreased precipitation and moisture availability, more severe minimum winter temperatures, and a shorter growing season that is probably linked to lower daily temperatures and higher snowfall. We cannot rule out the possible influence of humans who are more abundant in southern Maine and who also have a propensity to plant diverse trees and woody shrubs. In the case of the southern and northern range limits for some species, we can speculate that certain aspects of the environmental variations and climate gradients may impose stresses on more southerly species that are intolerant of extreme cold events or the shortened frost-free season encountered moving northward. Similarly, more

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northerly or boreal woody species may be stressed by summer extremes of temperature or drought, and this may contribute to the observed limits at the southern end of their range.

3.6 Influence of Geologic Factors in the Environment Touring the continental U.S., we can find striking examples of geologic formations at iconic locations such as Yosemite National Park, Bryce Canyon, and Yellowstone National Park that grab our attention and remind us of the ways in which geologic features can dominate the landscape. But, how often do we consider the importance of geology in living systems of the biosphere? In this section, we examine the properties of geologic materials that influence living organisms and ecological relationships. The rocks and rock fragments that surround us affect ecological conditions in at least five major ways  – as a source of nutrients, acid buffering compounds, substrates for soil formation, clay minerals, and as a physical matrix for organisms, water storage, and water movement. When minerals within a rock matrix break down through weathering processes, different combinations of vital nutrients can be released, including calcium, magnesium, sodium, potassium, iron, phosphorus, silica, sulfur, manganese, and a variety of other trace elements. Weathering by carbonic acid can produce ions such as HCO3− that act as proton acceptors and contribute acid neutralizing capacity (ANC) to drainage waters, ground water, and surface waters. This ANC helps to prevent or to limit acidification of soils and surface waters. During the processes of chemical and physical weathering, clays are generated that provide ion storage properties for soils, while additional silt and sand-sized particles are created that help to provide the porous matrix that is transformed over time into a soil substrate. One counter-point to these beneficial attributes of geologic materials is the fact that certain rock minerals contain substances that may act as toxins. For example, copper sulfide (CuS) and galena (PbS) are sulfur-bearing minerals that break down to release copper and lead ions that may be harmful to organisms. Let us look at one example of a weathering reaction to see how this process can provide a number of benefits for organisms in an ecosystem. As shown below, when a calcium-rich feldspar is exposed to water containing carbonic acid (derived from CO2), the weathering products include soluble Ca2+ ion, bicarbonate ion (which acts as ANC for acid buffering), and a kaolinite clay that acts as a source of cation exchange (nutrient storage) in the soil environment. Thus, just this one reaction provides a key nutrient element, acid neutralizing capacity, and a residual clay mineral that helps to conserve a storehouse of exchangeable nutrients for biological uptake.

CaAl 2 Si 2 O8  2H 2 CO3  H 2 O  Ca 2   2HCO3   Al 2 Si 2 O5  OH 4



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In the grand scheme of the biosphere, mineral weathering is a process that links geological sources with biological nutrient sinks; thus, this process is a key nutrient cycling pathway. Yet, it is important to note that not all geological materials are the same – they may contain nutrient-poor or nutrient-rich minerals and chemical compositions. Thus, one important environmental attribute to consider in an ecosystem analysis is the kinds of geological materials and associated minerals represented in a given study system. Let us examine two contrasting watershed ecosystems to explore how differences in geological materials can shape conditions for biological communities. Watershed A contains a mixture of calcium feldspar and limestone as the primary geological substrates. We can predict that weathering of these materials will produce a soil enriched in calcium, with ample acid neutralizing capacity and a high fertility for plant growth. In addition, drainage water flowing through this soil will be buffered by ANC (as HCO3−) and will be an excellent water source for aquatic life in lakes. In contrast with that first example, Watershed B contains granite overlain with quartz sand as the major geologic substrates. The resistant bedrock and nutrient-deficient quartz sand weather slowly, provide only limited soil nutrients for plant growth, and do not release much ANC. Drainage water flowing through this poorly buffered soil will tend to remain acidic and this acidity that may threaten aquatic organisms in lakes and streams.

3.6.1 Surficial and Bedrock Geology The geology in most ecosystems consists of a bedrock component that originates as part of the mantle of the earth and an overlying surficial geology. In a simple case, picture the sand dunes of the Sahara Desert as the surficial geology and the underlying “hard rock” material as the bedrock. Surficial geologic deposits represent the soil parent material for pedogenesis or soil formation in most ecosystems. Thus, the physical and chemical characteristics of sediments and particles in the surficial geology of a system can exert an important influence on soils and the associated life support characteristics of terrestrial ecosystems, as well as adjacent aquatic ecosystems. Let us compare the different types and origins of surficial geologic deposits that are commonly observed in the biosphere. In some ecosystems, surficial geology originates in situ, rather than being transported from elsewhere. As such, we can consider this type of geologic material as sedentary and derived from the local geology. A major example is bedrock residuum or saprolite, which originates as bedrock weathers in place and leaves behind sedimentary remains. In the Shenandoah National Park, VA, hiking trails lead you

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beside igneous or metamorphic outcrops that resemble hard bedrock, but are actually bedrock residuum that crumbles in your hands. Many of the surficial deposits in the landscape are derived from water-­ transported geologic materials. In floodplains and riparian zones bordering streams and rivers, we find alluvial or fluvial sediments that have been eroded from upstream channels and watersheds and have subsequently been deposited during flood events. These sediments are often fine textured silts or mixed particle sizes. In glaciated areas, there may be ancient sandy outwash deposits that originated in the delta area at the outflow of a glacial river. In contrast to those coarse-textured outwash sediments, lacustrine sediments are usually dominated by fine-textured silts and clays found in former lake beds where eroded sediments washed downslope into a lake and accumulated over the centuries and millennia in the benthic or bottom zone. Another important category is marine sediment that dates back to a period of ocean submergence of the landscape. During that ancient marine incursion, eroded fine-textured silts and clays would have accumulated on the ocean floor, along with the siliceous and calcareous remains of plankton. These later became exposed on the land surface when sea level receded, and now are part of the modern landscape. In regions where continental glaciation occurred, the landscape is likely to contain surficial sediments transported by ice and glacial meltwater. Glacial till is a deposit of unsorted sediments that can include clays, silts, sand, gravel, cobbles, and boulders. Looking across a glaciated landscape, you may see other deposits such as eskers (elevated ridges of coarse sediments derived from a glacial streambed) as well as moraines that resemble large berms or hilly arcs that originated at the terminus of a glacial advance. Wind is another transport agent that helps to shape surficial geology through erosion and deposition processes. Aeolian loess deposits are typically composed of fine silt particles that have eroded from one location, have been transported by prevailing winds, and have finally been deposited as a sedimentary blanket over the landscape. There is a large expanse of Aeolian loess in the Midwestern breadbasket of the United States where corn and wheat are major crops. In regions with a history of volcanic activity (e.g., in the Pacific Northwest near Mt. St. Helen’s), the surficial geology may reflect the strong influence of volcanic ash deposition. The fine-­ textured ash serves as the parent material for development of soils that are classified as Andosols. Sand dunes are yet another distinctive type of wind-transported surficial deposit that can be found in locations as diverse as the eastern shore of Lake Michigan, the Huacachina oasis in southwestern Peru, and the Sahara Desert. Dunes can be very active and unstable, and may be continually sculpted and shifted by prevailing winds. As such, dunes are much less likely to support soil development and associated colonization by plants. A surficial geologic map provides an integrated overview of the heterogeneous patterns of sedimentary deposits that occur across the landscape. As an example, the map in Fig.  3.4 illustrates how the State of Ohio contains a mosaic of glacial

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Fig. 3.4  Map of glacial geologic features in Ohio. (Ohio Division of Geological Survey 2005)

deposits, including outwash plains, eskers, moraines, and lacustrine or lake sediments. Because these surficial sediments vary physically and chemically, their properties can exert a strong influence on ecological conditions and interactions among organisms in a community. Before we leave this section, let us return briefly to the bedrock component of an ecosystem. In spatial terms, bedrock may occur as outcrops at the surface of the earth or as bedrock buried beneath surficial layers of soils and sediments. The deeper the bedrock, the less likely it is to have a direct influence on a local terrestrial ecosystem; instead, subsurface bedrock often interacts with and modifies the chemistry of ground water that drains into streams, lakes, and rivers. In this way, subsurface bedrock can indirectly help to determine the health and ecological relationships in aquatic ecosystems.

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3.7 Influence of Soils in the Environment Soils are a major determinant of site quality and productivity in terrestrial ecosystems, and differences among soils consequently have important implications for the abundance, composition, and interactions of species in a community. Soils affect not only terrestrial plants, animals, and microbes, but also aquatic communities that receive drainage water from soils in the surrounding watershed. The important differences among soils can be attributed to variations in one or more of the following five soil forming factors: (a) nature of the parent material; (b) climatic conditions; (c) age of the soil profile; (d) soil topographic position and drainage conditions; and (e) influence of biological soil-forming agents. At a given location, soil parent material may consist of glacial outwash, volcanic ash, or perhaps marine sediments, with each surficial deposit providing different physical and chemical properties for soil formation. Depending upon climatic factors such as temperature and moisture availability that vary across the landscape, a parent material located in one geographic location may experience a different pattern of soil formation as compared to a location with contrasting climatic conditions. Because soil formation is a relatively slow process, soil age is also a strong determinant of soil differences. This is one reason why ancient tropical soils are so different from more recent soils derived from glacial sediments in the temperate zone. The topographic position of a soil in the landscape influences whether a deposit of soil parent material becomes well-drained or poorly drained, and this can manifest itself in striking differences in soil development. A final major determinant of soil characteristics is biological activity in the form of plants, soil invertebrates, burrowing animals, and microbes, whose life processes, wastes, and detritus contribute to differentiation of soils in the landscape. Over time, soil forming processes transform geologic parent material into a distinctive soil profile that contains a vertical sequence of soil horizons or horizontal layers that differ in terms of visual, chemical, and physical properties. In Fig. 3.5, a generalized soil profile is depicted that contains O, A, B, and C horizons. In Fig. 3.5 Conceptual diagram of a soil profile in a temperate forest

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unplowed soils, the surface O or organic horizon typically contains a mixture of decaying leaves (litter or detritus) and humus, along with a dense accumulation of roots. Beneath that surface layer, a profile exhibits a number of mineral soil horizons derived from the underlying parent material. The uppermost mineral soil layer is usually either an A or and E (eluvial) horizon, where the symbol E corresponds to an A layer that has been so heavily leached that it turns from a brownish color to a gray or whitish color that is indicative of soil grains that have been stripped of their original nutrient content. Resting beneath the A or E layer are one or more B horizons that represent illuvial zones where metal ions and soluble organic matter derived from the O and A horizons accumulate as insoluble metal oxides and organic matter coatings on soil particles of the B horizon. B horizons may also be enriched in clays that contribute to reversible storage of exchangeable cations. Finally, the lower mineral profile grades into a C horizon that represents original soil parent material that has not yet been substantially altered by soil forming processes.

3.7.1 Soil Site Quality Given the importance of soils as a key environmental determinant, we need a conceptual framework for judging the physical and chemical quality of these substrates (sometimes referred to as site quality). Site quality influences not only the distribution patterns of natural communities and species, but also their productivity. We generally expect plant production to increase with improving site quality. There are two primary factors that determine site quality – nutrients and moisture. Poorer sites may be limited by one or both factors, whereas better sites are less likely to be limited by either site factor. For any given climatic or topographic setting, the nutrient and moisture supply characteristics of a soil are a function of several measurable physical-chemical properties: soil texture and structure, organic matter content, ion exchange capacity, fertility, acidity, and mineralogy of the parent material. In ecology, we are interested in how these parameters vary in the landscape and set the stage for different ecological outcomes. Soil texture is defined as the relative proportions of three size classes of soil particles: sand (diameter  =  0.05 to 2.0  mm), silt (0.002 to 0.05  mm), and clay (90% of winter wolf kills were elk. Not only did elk herd density decrease through mortality associated with wolf predation, but the heavy predation also helped to drive behavioral changes, with elk moving out of the valleys and into hillslope habitats to escape from wolf predators. With fewer elk at lower elevations, grazing pressure on vegetation declined and tree

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and shrub species recovered in the stream valleys. The dense plant cover stabilized stream channels against erosion and provided habitat for a variety of new species. The trophic cascade associated with wolves rippled through populations of scavengers (e.g., coyotes and ravens) who would typically depend on more severe winters to provide carrion as a food resource. Wolf kills helped to subsidize scavengers, providing a more constant supply of carrion in milder winters and throughout the year. In summary, the re-introduction of the wolf top predator severely decreased elk herbivore density, which generated a trophic cascade that allowed re-vegetation of the valley habitats that had been heavily browsed by elk, changed habitat conditions for other species, stabilized stream channels, and also influenced a variety of scavengers and competing predators. Thus, environmental conditions, habitats, and multiple trophic levels were affected. The location of the second example is the Aleutian archipelago in Alaska where seabirds nest on the many small islands and forage at sea for fish. In this story, fur traders introduced arctic fox to the islands in the hopes of harvesting pelts from the resulting fox populations. What happened in the aftermath of the introduction was that the fox acted as a keystone predator and set in motion a trophic cascade that affected birds, soils, and plants. Initially, arctic fox predation on nesting seabirds produced an obvious effect  – bird density declined steeply. The unexpected outcome of the bird decline was a reduction in nutrient transport from the ocean to the islands in bird guano. Over time, this loss of nutrient input contributed to lower soil phosphorus availability and reduced soil fertility, which contributed to less productive plant communities and altered habitat conditions. Thus, introduction of the fox predators affected not only their seabird prey, but also the plant community at the base of the food web through unexpected changes in soil fertility. As such, this is an example of a complex trophic cascade involving a keystone predator that preyed directly on seabirds, but also indirectly affected other trophic levels and environmental conditions (Croll et al. 2005).

5.5 Analysis of Food Webs with Stable Isotopes In a simple lake food chain, it may be relatively straight-forward to identify the organisms that belong to the primary producer, herbivore, and predator trophic levels. However, many food webs are sufficiently complex that it can be very challenging to determine the roles of different consumer organisms in the feeding relationships of the community. Is Species A or B acting as an herbivore, omnivore, carnivore, or detritivore, and is it feeding on one or multiple trophic levels? Although ecologists have long relied on analysis of stomach contents and scat (fecal pellets) to answer these kinds of questions, it is becoming increasingly common to use the power of stable isotope analysis to make inferences about trophic relations in different communities. Stable isotopes can be briefly described in the following terms. For any given element, the atomic mass is the sum of the number of positively charged protons and

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uncharged neutrons. Atoms of any element may include lighter or heavier isotopes with different numbers of neutrons and corresponding atomic masses. Each of these isotopes can be represented in chemical notation by writing its chemical symbol, preceded by a superscript that specifies isotopic mass. For example, an oxygen nuclide composed of 8 protons and 8 neutrons is written as 16O, whereas an oxygen isotope composed of 8 protons plus 10 neutrons is represented as 18O. Out of a total of 1700 known nuclides for all elements, roughly 260 of these are stable isotopes; elements may also occur as unstable nuclides that are subject to spontaneous disintegration over time. As an illustration, the element carbon occurs in two stable isotopic forms (12C and 13C), as well as the radioactive isotope 14C. Various biotic and abiotic processes can separate light and heavy isotopes of a particular element. For example, evaporation of water favors enrichment of the vapor phase with the light isotopes of hydrogen and oxygen, leaving residual liquid water enriched in heavier isotopes. This mass fractionation of light and heavy isotopes allows scientists to trace the pathways and processes of element cycling by tracking changes in the ratio of heavy to light isotopes for a given element (e.g., 13C versus 12C, 15N versus 14N, or 18O versus 16O). It is important to note, however, that the absolute changes that are measured are very small, because heavy isotopes represent a small percentage of the isotopic pool for a given element. In terms of natural abundance, the heavy isotope 13C represents 1.1% of the carbon pool, 15N is roughly 0.37% of the nitrogen pool, and 18O is approximately 0.2% of the oxygen pool. Using a sensitive mass spectrometer, small fractional changes in abundances of these relatively rare heavy isotopes can be detected and used to trace biogeochemical and ecological patterns and processes. Let us try to establish a bit more familiarity with the application of stable isotope chemistry to ecology. If a biological process discriminates against a heavy or light isotope, isotope fractionation occurs and the resulting product becomes depleted or enriched in heavy isotope. Suppose the heavy isotope normally represents a concentration of 3 parts in 1000 in an organism; after fractionation, the heavy isotope in a tissue sample might equal 2 parts in 1000. This sample would then be considered depleted in the heavy isotope and would therefore be considered isotopically lighter (Fig.  5.9). Alternatively, a biological or physical-chemical process could favor enrichment of the heavy isotope in the tissues of an organism, resulting in a final concentration of 4 parts per thousand, which would be termed enriched or isotopically heavier. Mass fractionation of stable isotopes is expressed in delta (∂) notation that represents the ratio of heavy to light isotope in a sample (Rsample) as compared to the same ratio in a reference standard (Rstandard). The value of ∂ is essentially a measure of the deviation of Rsample from Rstandard in parts per thousand (‰). For example, a biological specimen might have a value of ∂ 13C = (Rsample / Rstandard − 1) * 1000 = −20‰. We interpret this to mean that the specimen sample is depleted in heavy 13C by 20 parts per thousand as compared to the standard reference material. Hence, some fractionation process has discriminated against the heavy carbon isotope. When we are presented with stable isotope data, samples with positive values of ∂ are isotopically heavy or enriched, whereas samples with negative ∂ values are isotopically light and

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Fig. 5.9  Graphic depiction of stable isotope depletion or enrichment

are depleted in the heavy isotope. As we shall soon see, sometimes it is a matter of comparing biological samples that are more depleted (e.g., −20‰) with those that are less depleted (e.g., −9‰). How might we apply this technique to ecological problems? One of the interesting features of nitrogen metabolism and assimilation is that the heavy isotope 15N becomes enriched by 3–4‰ at each trophic level moving from the base of the food chain to the top predators (Fig. 5.10). Possible explanations for this observation are that heterotrophic consumers favor heavy 15N in protein formation or preferentially eliminate light 14N during excretion. This pattern can be used to make inferences about the feeding relationships of different organisms. Thus, if plants at the base of the food chain have a ∂ 15N value of +6‰ and a consumer in the community exhibits a tissue ∂ 15N value of +12‰, it is likely the organism feeds at least two trophic levels above the plants and one level or more above the herbivores. Carbon isotopes provide additional insights for some food webs, because plants discriminate among 12C and 13C isotopes to different degrees, depending on whether the plant has a C-3 or a C-4 photosynthetic pathway. C-3 plants such as trees produce biomass that is strongly depleted in 13C, with ∂ values in the range of −25‰. In comparison, C-4 plants including grasses produce biomass that is isotopically heavier, with ∂ values in the range of −15‰. These isotopic differences allow one to infer the degree to which a consumer is dependent upon primary production derived from either C-3 or C-4 plants. As a final step, we see in Fig. 5.11 that organisms in a community can be plotted on a diagram showing tissue ∂ 15N values on the y-axis and ∂ 13C values on the x-axis. On the vertical axis, organisms separate into different trophic levels based on

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Fig. 5.10  The relative abundance of 15N isotope increases roughly +3‰ from plant to herbivore to carnivore trophic levels

Fig. 5.11  Conceptual map showing the members of a food web positioned in relation to their isotopic signatures of 15N and 13C

the understanding that ∂ 15N values increase by +3 to 4‰ for each step in the food chain. On the horizontal axis in this illustration, the base of the food web contains both C-3 and C-4 plants, which means that consumers will exhibit tissue ∂ 13C values that reflect a diet based on C-3 plants (more depleted), C-4 plants (less depleted), or a combination of both. The diagram and stable isotope data therefore provide a way to elucidate the trophic position and food sources of each species in the food web. In a recent study, investigators applied this approach to understand the diet of black bears in Yosemite National Park. They hypothesized that bears have three major sources of food – fish, human kitchen waste, or a mix of native plants and

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animals. Using archived and current hair samples from bears, the scientists found that YNP bears relied heavily on human food when local dumps were prevalent, but that after dump closures, bears diets shifted toward a more natural and plant-­ based diet. In closing this last section, let us consider a field sampling scenario. You are an animal ecologist and want to know if a small mammal you are studying is feeding in a corn field or an adjacent forest. You sample hair from the mammal and analyze for ∂13C and ∂15N. Which of the following two isotopic values of ∂13C would you expect if most feeding was in the corn field: −25‰ or − 15‰? If the plants in the study area have a ∂15N value of +3‰, which value of ∂15N is more likely in the small mammal: −3‰, 0, +3‰, or +6‰?

Chapter 6

Landscape Ecology and Conservation Biology

You may have heard the song Big Yellow Taxi by Joni Mitchell, and perhaps recall the haunting poetic lines: You never know what you’ve lost, ‘til it’s gone. They paved paradise and they put up a parking lot. Believe it or not, she was describing a major issue in landscape ecology  – human land development. It is one of a number of important issues in ecology and environmental science that transcend the landscape mosaic of habitats, communities, and ecosystems that are at risk from diverse regional, continental, and global threats and stressors. The words of the song are applicable in southern California, in the Amazon Basin, in central Africa, and almost anywhere that human settlements are expanding. Modern humans have been very busy transforming undeveloped open space such as forests into a developed landscape dominated by housing, commercial construction, intensive agriculture, roads, and other infrastructure. In one example from the African continent, consider the migratory herds of wildebeest and elephants that traverse the plains of Tanzania and Kenya. Increasingly, these wild populations are at risk from human development activities, farming, border fences, and roads that act as barriers and threats to historical migration routes for these creatures and other wildlife. The future of these at-risk species is uncertain and solutions will require a delicate balance between wildlife conservation concerns and human socio-economic priorities. Should future land use decisions favor human development or protection of wild populations? Given the complexity of wildlife conservation and land use issues, are there any organizing principles to guide us toward meaningful answers and solutions. Indeed, there are some possibilities! This chapter examines how concepts of landscape ecology and conservation biology offer a framework and perspectives for analyzing and addressing problems and contentious issues arising at the interface of our expanding human population and the natural world. Two primary questions will guide our analysis: (i) is biological diversity at risk from human activities in the biosphere; and (ii) how do we limit the footprint of human activities to minimize adverse impacts on biological diversity and ecological integrity?

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. S. Cronan, Ecology and Ecosystems Analysis, https://doi.org/10.1007/978-3-031-45259-8_6

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6.1 Concepts of Conservation Biology Biodiversity refers to the diversity of life in all its forms and levels of organization, and conservation biology is a discipline that focuses on preserving biodiversity, with a major emphasis on protecting species from extinction. As such, conservation biologists devote considerable attention to endangered species that are likely to become extinct throughout all or a large portion of their range, threatened species that are likely to become endangered in the near future, and critical habitats that are vital to the survival of endangered or threatened species. Four common terms in the vocabulary of conservation biology are endemic species (which are found in a defined geographic area); extinction (disappearance of a species from earth); extirpation (small-scale extinction from a region); and charismatic megafauna (e.g., lions, tigers, bears that attract public attention). International conservation organizations track the status of threatened and endangered species using the IUCN categories listed in Table 6.1. Extinction is a very real phenomenon, but is exceedingly difficult to measure. Similarly, our estimates of global biodiversity are highly uncertain. It is generally accepted that we need to minimize extinctions and to maximize the preservation of biodiversity, but keeping track of our progress is challenging, to say the least. Costello et al. (2013) estimated that the global total number of species is 5–8 million, with the majority of those yet to be described. Other scientists have suggested much higher estimates of global diversity. Rates of species loss or turnover have been estimated to range from an historical value of 1 species per million species or 0.0001% per year to a modern rate perhaps 100–1000 times higher. What is striking is the fact that over a period of only 100 years, a modern extinction rate of 0.01–0.1% per year could potentially yield a loss of 1–10% of biodiversity in a time span of a century, which is a blink of an eye in geological terms. Botanists estimate that 13% of the global flora is threatened with extinction. However, a study by Pitman and Jorgensen (2002) calculated much higher estimates ranging from 22% to 47% of global flora. In another investigation, Sinervo et al. (2010) used physiological models, climate projections, and field data to estimate that by 2080, global warming will

Table 6.1 Conservation categories for species at risk based on guidelines from the International Union for Conservation of Nature (IUCN)

Extinct Extinct in the wild Critically endangered (50% probability of extinction within 10 years) Endangered (20% probability of extinction within 20 years) Vulnerable or threatened (10% probability of extinction within 100 years) Conservation dependent

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push 39% of local lizard populations and 20% of global lizard species to extinction. Unfortunately, there are many uncertainties and assumptions that make it difficult to know how extinctions rates will actually unfold, but the trends demand our attention! In the U.S., there are approximately 18,400 known plant species and roughly 5% of those are listed as threatened or endangered by the U.S. Fish and Wildlife Service. However, Wilcove and Master (2005) estimate that the total percentage of U.S. vascular plants at risk of extinction is over 15% of the total. Examples of endangered plant species include Western Lily (Lilium occidentale) in northern California, Bakersfield Cactus (Opuntia basilaris var. treleasei) in California, the Florida golden aster (Chrysopsis floridana), Kearney’s Blue-star (Amsonia kearneyana) in Arizona, the Dwarf Bear-Poppy (Arctomecon humilis) in Utah, Mead’s Milkweed (Asclepias meadii), Golden sedge (Carex lutea) in North Carolina, Smooth Coneflower (Echinacea laevigata) in the eastern U.S., and Rose mallow (Hibiscus dasycalyx) in Texas. One of the challenges in conservation biology is to identify the locations of threatened and endangered species, so that efforts can be made to protect and to sustain them. In North America, the locations and densities of endangered plant, fish, bird, and mollusk species were mapped by Dobson et al. (1997), and the results provide a glimpse of the extent and heterogeneous distributions of at-risk species. Counties with the highest numbers of endangered plants and fish are predominantly found in the southwestern U.S., whereas counties with elevated numbers of endangered freshwater bivalve mollusk species are concentrated in the eastern U.S. In the avian arena, counties with higher numbers of endangered bird species are distributed along the west coast, the southeastern coast from Virginia to Florida, and down the mid-western spine of the U.S. from Minnesota to Texas. There is plenty of evidence that species are increasingly at risk across the biosphere, but what are the root causes and factors that are threatening biodiversity and pushing species toward extinction? At or near the top of the list of threats are land use changes in the form of habitat degradation, destruction, and fragmentation by human activities. Without suitable habitat, species risk dropping below minimum viable population sizes required for long-term survival. Compounding the threat of land use changes is the stress of global climate change that is gradually shifting bioclimatic zones, encroaching on coastal habitats through rising sea level, and altering the timing of life cycle events that depend upon synchrony between plants and animals (e.g., pollination). In many different locations, species are at risk from over-exploitation in the form of hunting, harvesting, and collecting. As an example, it is predicted that African rhino populations may become extinct in the wild in 15 years, if poaching continues to accelerate along current trend lines. Elephants are part of another interesting example. After poaching pressure for ivory tusks threatened African elephant populations, trade restrictions were imposed to curb the illegal sale of contraband ivory. One unexpected outcome of this policy was that as elephant tusks become rarer on the international market, a new ivory emerged in the form of the “jade of the sea” from giant clams that are an endangered species in their own right. Thus, protection efforts for elephants simply shifted the threat to another species.

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With rising numbers of invasive exotic species appearing as transplants from their native region (e.g., Europe) to entirely new continents (e.g., North America), there is broad-scale concern and increasing evidence that these aggressive introduced species will serve as yet another threat for less competitive native species, pushing them toward endangered status. For instance, the zebra mussel (Dreissena polymorpha) has invaded waterways in the eastern U.S. and has caused dramatic declines in many of the naturally occurring freshwater species. In Hawaii, the endemic coral tree is at risk of extinction as a result of an invading Asian parasitic wasp. Species are also at risk from a variety of emerging infectious diseases, including rinderpest and canine distemper in Africa, Varroasis in bees, mammalian sarcoptic mange, avian influenza, and chytrid fungi. Over the past 30 years, emergence of a globalized pandemic of chytrid fungi has caused serious declines of amphibian species in Central America, Europe, Australia, and North America. A final risk factor stems from the size of a declining species population. Species with ever smaller population sizes and shrinking gene pools are at risk from a loss of genetic diversity, genetic bottlenecks, and inbreeding in small populations. A genetic bottleneck occurs when a small remnant of a species with a correspondingly diminished gene pool recovers through conservation efforts, but then suffers from a lack of genetic diversity in subsequent generations.

6.2 Concepts of Landscape Ecology and Land Use Change As indicated in the preceding section, a key issue in both landscape ecology and conservation biology is the loss and fragmentation of habitats associated with land development, as well as the impairment of aquatic ecosystems that often accompanies human development. Imagine returning to your childhood home and finding that the farm across the road has been turned into a shopping mall or the forest land behind your house has been cleared to make way for a subdivision filled with new houses! Perhaps you can also imagine losing access to your favorite lake where you used to paddle, swim, or fish, but where the shoreline is now private lakefront property. Sooner or later, land development activities will impact our personal lives and help to sensitize us to the larger issue of land use change. Land use changes have become virtually inevitable in our modern world, and where they occur, these changes can potentially diminish our quality of life, create disturbances and stress for sensitive species, and even push some species toward endangered status or extinction. Given the potential repercussions of unsustainable land development, there are several questions we should be asking regarding the future. (i) How much undeveloped open space do we need? (ii) How much development and habitat fragmentation can we afford? (iii) How can we identify the balance point that we can live with? (iv) How can we achieve that balance point? (v) Finally, can ecological principles provide objective guidelines for decision-making on this issue? For some perspective on the first question (i) above, we can look at three examples. In New York City, with its canyons of pavement and skyscrapers, roughly 7%

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of the city is set aside as public park open space. In the State of Maine, almost 20% of the land area is protected as public open space. By comparison, the Wildlands and Woodlands project at Harvard Forest, MA has proposed that 70% of New England be protected as undeveloped forest land. There is thus a wide range of actual and proposed values for a desirable level of open space conservation. As we might expect, there are many diverse opinions on the necessity and role of open space and these divergent perspectives reflect different priorities with respect to economic, social, and ecological values. One of the major sources of polarizing disagreement is the issue of private versus public ownership of land. Some would argue that owners of private land should be allowed to use the land as they see fit and to develop any or all of the land without regulation or interference from the public. Others argue that there are important public goods associated with open space that must be protected from unregulated land development. Beneficial open space attributes include the provision of ecosystem services in the form of clean air and water, wildlife habitat, timber and food production, watershed and aquifer protection, and outdoor recreation. In the U.S., there is a long history of court cases focused on the tension zone between the protection of open space and individual property rights. It sometimes helps to have a visual image of what is meant by human development and the processes of urbanization. Have you ever walked through a dense residential development that has replaced a parcel of former open space with pavement and cheek-by-jowl housing? Figure 6.1 illustrates a 30 year time span during which large undeveloped open space parcels in a town in southern Maine were transformed into residential neighborhoods containing dense rows of small house lots. Patterns similar to these can be found across the urbanizing world and are generally to be

Fig. 6.1  The diagram shows a town that experienced rapid build-out of rural land (left) into suburban house lots (right) over a 30-year time span

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expected, given the way in which land use decisions are often made. Many towns essentially lay out the “game board” that assigns open space to zoning districts, and then the process of real estate development is allowed to unfold with minimal oversight, largely in response to market economics and consumer demand. With this conventional approach, open space often is what is left over, usually in the form of wetlands and lower value fragmented habitat. As such, concerns with biodiversity are typically not even considered at the local level where open space is at risk. Given our prevailing somewhat haphazard approach to human development, is it possible to envision an alternative “smart-growth” approach in which development and conservation decisions are based on a shared strategic planning process? Is there a pathway that could produce a balance between complete build-out of existing open space versus complete preservation of the undeveloped land base? One way to forge an improved public policy for land use decisions is to begin with an effort to identify the guiding ecological, economic, sociological, and constitutional principles and tools that can offer a basis for smart growth and sustainable development. An important first step in the process of setting ecological guidelines is to inventory the natural resource base of a town or other jurisdiction in order to set conservation priorities for the different attributes or features in the landscape. With a resource inventory completed, citizens and decision makers can work together with the assistance of conservation professionals to evaluate and to rank the presence and importance of landscape features such as those listed in Table  6.2. For example, if biodiversity is ranked as an important value, a jurisdiction may choose to focus on conserving as much habitat land area and habitat diversity as possible to minimize threats to species and genetic diversity, or it may be desirable to protect specific habitats of endangered or threatened species. In a case where ecological integrity is selected as a high priority, an emphasis may be placed on protecting intact, un-­ fragmented, functional, and healthy ecosystems (e.g., an undeveloped watershed) with their associated diversity of species, genetic resources, and “ecosystem Table 6.2  Examples of ecological attributes for ranking conserved open space

Ecological diversity and/or presence of rare species Physiographic diversity Degree of human disturbance and ecological integrity Size of the resource or habitat and corresponding suitability for particular uses Connectivity to corridors + other conservation lands Watershed considerations related to water quality Value for recreational trails

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services.” As citizens and decision makers grapple with the task of setting targets for open space conservation, a variety of questions may arise, including: (i) how large must nature preserves be to protect species; (ii) is it better to create a few large reserves or many smaller reserves; and (iii) should reserves be isolated or connected? With a resource inventory completed, the next step in the process is to consider legal or constitutional perspectives and issues. At the present time, there is a robust history of case law on land use issues and public regulations aimed at guiding, managing, or limiting land development without infringing on or “taking” the property rights of individuals; these legal precedents help to frame the boundaries for land use plans. Increasingly, towns are adopting regulations that require developers to follow conservation planning principles that may include compulsory open space set-asides in new residential subdivisions, clustering of house lots to minimize the footprint of development, or allowance for the transfer of development rights as a means of protecting valuable natural resources while offering developers the ability to increase housing density in a parcel with lower conservation value. For a smart growth strategy to be acceptable to tax payers, it must incorporate a realistic and fair set of economic guidelines or principles. This usually means that everyone has to give a little. In some cases, a town may offer tax incentives for conservation of land by private owners. If that approach fails, one of the most direct ways of overcoming the economic hurdle of land conservation is to use public tax money or non-profit land trust funding to acquire open space from private land owners who are willing to sell their property for open space conservation, rather than residential or commercial development. One interesting alternative to the direct purchase option is to use a conservation easement, which allows a land owner to sell the development rights on a piece of land while retaining ownership of the property. Under this scenario, the land owner receives payment for signing a legal agreement to extinguish all future rights to develop the property, and the buyer is able to insure long-term conservation of the land at a price that is less than outright purchase of the property. Yet another approach that borrows from the age-old concept of “horse trading” is the transfer of development rights (TDR) that was mentioned in the previous paragraph. An example would be an instance where a person owns two land parcels, one of which is very suitable for development whereas the other has intrinsic conservation value. In the current land use zoning plan, each parcel may have an allowable housing density of 1 dwelling per acre. A town that wished to protect the conservation parcel could negotiate with the owner to transfer some or all of the allowable development rights from the conservation parcel to the buildable parcel, so that housing could be clustered at a higher density on one parcel, while the other parcel would be conserved and unavailable for future development under the legal terms of the TDR agreement. To summarize this section, there are two major steps in the design and implementation of a strategic land use plan. (1) Inventory the landscape and use criteria based on public input and expert knowledge to identify land and natural resources that are most suitable for conservation, human development, or working farms and forests and then rank the conservation priorities by consensus. (2) Establish a legal, regulatory, and economic framework to guide future growth toward land parcels that

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are most suitable for development and to protect high value conservation lands and working farms and forests.

6.2.1 Rank Your Personal Conservation Priorities To encourage critical thinking on the topic of land use, let us take a few moments to evaluate a scenario called the 50% Solution, which takes place under the following conditions. Your town contains a mix of residential housing, fields, farms, forests, wetlands, ponds, streams, a lake, trails, a commercial center, a road system, and a public park. Most land is privately owned, but there is interest in raising funds to conserve part of the town for the future. Let us assume that only 50% of the undeveloped land in the town can be conserved. What are your conservation priorities? What criteria or principles will guide your choices and priorities? In this kind of scenario, there is no single correct answer; rather it is a matter of forming a coherent and logical proposal that can be weighed against other competing perspectives reflecting the many diverse philosophies on the topic of land conservation. As you consider how to identify your own personal key conservation priorities, you may wish to emphasize one or more of the following ecological themes: (i) preserve biodiversity; (ii) maximize habitat or physiographic diversity; (iii) protect specific habitats (e.g., riparian zones along rivers or locations of threatened or endangered species); (iv) maintain biological and ecological integrity of intact habitats, watersheds, or ecosystems that are at risk of fragmentation; (v) protect natural resource features that provide valuable ecosystem services (e.g., wetlands that buffer against storm erosion and flooding); (vi) maintain connectivity between conserved areas with trails and undeveloped corridors; and (vii) provide open space for human outdoor recreation. It may be helpful to ask yourself how this landscape is likely to change in the future in response to population pressure and market forces. What will be lost as open space is developed and will these losses be acceptable?

6.3 Designing an Action Plan for Conservation Biology We now have a sense of the threats to biodiversity and recognize that land use changes are one of the important risk factors for the rich diversity of species and habitats in the biosphere. Before we proceed to the question of what do we do about it, let us first remind ourselves why biodiversity matters. Some would argue that caring about biodiversity is rooted in human aesthetic and moral values – the beauty of the Earth depends upon biodiversity and humans are responsible for stewardship of all forms of life. An ecologist would argue that biodiversity provides an ecological buffer, serves as a source of resilience and resistance against unknown future stresses and disturbances, and represents the raw ingredients for food webs and

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healthy ecosystems that provide essential services for human welfare. Besides that, there are numerous examples of species that have contributed to unexpected scientific breakthroughs, and this reminds us that a species which currently appears to be of little consequence for humanity may ultimately be critically important for a new medical cure or other key application. A final consideration is that the production of plants at the base of the food web benefits from biodiversity. As an example, grassland experiments by Tilman et al. (2001) demonstrated that plots with 16 plant species produced 2.7 times the biomass of monocultures based on a single crop species. Today, we are playing a game of catch-up trying to develop a plan for curbing the threats to biodiversity and sustaining global biodiversity into the future. Conservation biologists have addressed this challenge with a number of approaches. A first step is to examine the natural history and ecology of target species and to use that information to guide management plans. Given the positive relationship between land area and species diversity illustrated in Fig. 6.2, efforts have been made to establish and to expand the land base of ecological reserves or protected conservation areas that offer intact habitat resources for a wide range of species. Using this so-called coarse filter approach, conservation agencies have been able to protect a land area covering 13% of the Earth. One of the questions we might ask a conservation biologist is what factors are considered in deciding the locations and features of ecological reserves and protected areas? What we would find is that the answer varies with the goals, funding resources, ecological constraints, and available land area for a particular project. To illustrate the complexity of selecting a site for a conservation project, we can refer to a study by Thiollay (1989), who analyzed home range requirements for conservation of tropical forest raptors (“birds of prey”). He examined eleven 2500 ha plots of primary tropical rain forest in French Guiana and observed 27 species of forest raptors including eagles and falcons. In his study, 80% of the total set of species occurred in the first two plots representing 5000  ha of the sampled forest area. However, the remaining 20% of raptor species had such low densities and large Fig. 6.2  A version of the species-area curve illustrating that species diversity benefits from increasing the area of protected lands

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ranges that they required an area four times larger (20,000 ha) for conservation of their habitat. A conservation agency would welcome these important natural history insights as a guide for protecting tropical forest raptors, but they would face a potentially serious dilemma – could they raise enough funding and support to purchase and to protect the 20,000 hectares necessary to encompass the home ranges of all 27 species or would their financial capital fall short and limit their ecological reserve to 25% or 50% of the total? Besides the coarse filter approach to land conservation, there are many instances where an alternative fine filter approach is implemented to protect specific habitats of threatened or endangered species (e.g., old growth forests that provide habitat for spotted owls). One secondary benefit of this approach is that the threatened or endangered animal or plant can act as an umbrella species – in other words, its protected habitat also serves the needs of many other non-target neighbor species. Another strategy in conservation biology depends upon using field survey data from around the world to identify biodiversity hot spots that offer unusually rich species diversity. In these locations, lands that are set aside as protected habitat provide an extra conservation benefit as compared with conventional land areas with lower overall biodiversity. Thanks to the availability of satellite remote sensing data and digital GIS mapping technology, methods of GAP analysis are being used to identify unprotected habitats and species. In this technique, locations of critical habitat for threatened and endangered species are mapped and are then compared with maps of conservation areas to determine whether critical habitats are adequately represented in the current mix of conservation lands or if there are gaps in coverage. As an example, the blue dots on the map of Maine in Fig. 6.3 indicate that many bald eagle nests are located outside of conservation lands, which implies that there are gaps in the distribution of protected habitat for this iconic species. GAP analysis can be used to guide the selection of future targets for land conservation. There are three major tools that are designed to address the challenges of species that have reached the threshold of threatened or endangered status: (i) the legal shield of the Endangered Species Act (ESA) can be invoked; (ii) critical habitat can be rehabilitated through restoration ecology, and (iii) population recovery can be facilitated through captive breeding. The federal Endangered Species Act is designed to protect species in the USA from extinction as a consequence of economic growth, development, or other human influences. Under the authority of the ESA, a species may be listed as either endangered or threatened, and the listing process includes a recovery plan describing the steps needed to restore a species to ecological health. The act is designed to protect both the species at risk and “the ecosystems on which endangered species and threatened species depend.” Thus, habitats as well as endangered species are protected, and the ESA requires the designation of “critical habitat” for listed species when “prudent and determinable.” According to the U.S. Fish and Wildlife Service, critical habitat is defined as geographic areas that contain the physical or biological features that are essential to the conservation of a listed species and that may need special management or protection.

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Fig. 6.3  Example of a GIS-based GAP analysis – blue dots indicate that many bald eagle nests are located outside conservation lands (depicted in green). (From Cronan et al. 2010)

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In cases where critical habitat has been degraded, it may be necessary to use techniques of restoration ecology to enable recovery of a species. For example, a stream that has been degraded and no longer supports an endangered fish species may require restoration of shady forested riparian zones and the addition of coarse woody debris (e.g., logs and branches) to provide suitable habitat conditions in the stream channel. For endangered species that have declined to minimal population levels, biologists may have to bring remaining individuals into captive breeding facilities where population recovery can be encouraged in the absence of environmental threats. Take the example of the black-footed ferret in the western U.S., a species that was near extinction in the 1970s. In 1985, a captive breeding program was initiated with 6 ferrets, but they all died. Two years later in 1987, a second effort at captive breeding was launched with 18 remaining ferrets, and that trial fortunately succeeded, resulting in the production of over 3000 ferrets that were re-­ introduced in Wyoming, South Dakota, Montana, and Arizona.

6.4 Moving On We have just scratched the surface of interesting ideas and issues in the fields of conservation biology and landscape ecology. Hopefully, readers will find a personal connection to the issues of biodiversity and land use change, and will discover ways to have a positive impact on efforts to sustain biological diversity and to implement more sustainable approaches to human development. In our own studies, my students, colleagues, and I have used GIS methodology and computer spatial models to evaluate the success of past landscape conservation programs, to identify parts of the regional land base that are most suitable for conservation or development, and to identify streams and rivers that are most sensitive to the threats of future urbanization in the landscape.

Chapter 7

Forest Ecosystems

When you hike into a forest community, what is it that first strikes your senses and captures your attention? Is it the diversity of species, the architecture of the forest canopy, the shady cool micro-climate, the quiet chatter of foraging birds, the smell of decaying leaves, or the splashes of light and color in the sun-flecked understory? In this next section, let us take these familiar images of a forest and explore four focal questions: (i) what are some of the key ecological interactions in forest ecosystems; (ii) is there observable evidence of these ecological activities and processes; (iii) what metrics can be used to describe and to compare ecological conditions in forest ecosystems; and (iv) how is a forest community influenced by different biotic and abiotic factors within an ecosystem? To begin, we should take a moment to develop a brief geographic overview and historical perspective on forest ecosystems.

7.1 A Geographic Perspective On a global basis, forests cover approximately one third of the land surface and account for almost half of net primary production of plant biomass. The geographic limits of forest distribution are largely set by climatic factors (precipitation and temperature), availability of soil, fire frequency, and human land use patterns. If the climate is too cold, too dry, or the growing season is too short, trees may be excluded from a region. Forest cover types range from the very productive and biologically diverse tropical lowland evergreen forests of the equatorial zone containing hundreds of tree species per hectare, to the relatively slow growing boreal taiga forests of the subarctic region containing as few as 5–10 tree species per hectare. Between these distant tropical and boreal forest endpoints, many other major forest types are

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. S. Cronan, Ecology and Ecosystems Analysis, https://doi.org/10.1007/978-3-031-45259-8_7

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distributed across the biosphere, each displaying a particular pattern of biodiversity and productivity. One example of a forest cover type is the northern hardwood forest that extends across the northern tier of the eastern U.S., through southern Ontario and Quebec, into the Canadian Maritimes, and pushes southward along the Appalachian Mountain chain. The overall floristic composition of tree species in the northern hardwood forest includes a mix of species whose ranges partially overlap, including American beech, sugar maple, yellow birch, white birch, red maple, black cherry, basswood, white ash, northern red oak, white pine, balsam fir, red spruce, and eastern hemlock. Of course, not all of those species are distributed evenly across the northern hardwood region – for any given location, the forest community includes only those species that are compatible with local site conditions and the successional status and history of the forest. A final note is that the softwood conifers on the preceding list are not primary components of this forest cover type, but are nevertheless present in limited numbers in many stands.

7.2 History of the Forest Landscape A forest ecosystem is not static – it has a history, a present condition, and a dynamic future. Even as we pause beneath a beech-maple-birch canopy in a northern hardwood forest community, change is slowly occurring … on the time scale of days, months, years, centuries, and millennia. Before this location became a northern hardwood forest, some other biological community flourished at this site. The stone wall running through the forest may reflect a prior period of farming on this same piece of land; then, after agricultural abandonment, the land gradually reverted back to forest cover. Other changes have preceded and followed that short segment of land use history and have exerted varying influences on the forest ecosystem we see today. But, what if we could look back into the past to gain a longer-term perspective on the landscape history and changes that occurred on a given piece of land where we now see a northern hardwood forest or some other forest cover type? As it turns out, the tools of paleoecology – analyzing fossil plant and pollen remains – provide a means of reconstructing vegetation and climate history, so that we can understand how contemporary forests fit into an historical progression of landscape changes. For purposes of illustration, let us apply this approach to the New England landscape that now supports northern hardwood forests. Like much of the North Temperate Zone, the New England landscape has been transformed over time by glaciation and changing climates. Since the last glacial retreat in North America (~14,000 B.P. “before the present”), revegetation in New England has followed a pattern and sequence recorded in pollen remains that have been analyzed by contemporary ecologists. At 13,000 B.P., arctic tundra vegetation covered most of New Hampshire, Vermont, and parts of Maine. The remaining portions of Maine were either beneath sea level or were still covered with the retreating ice sheet. By that period in time, mixed woodland vegetation had only migrated

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northward to the southern tips of New Hampshire and Maine. Following another 2000 years of deglaciation at roughly 11,000 B.P., mixed woodlands and coniferous forests covered most of New Hampshire, Vermont, and the southern two-thirds of Maine. The cold northern portion of Maine was dominated by arctic tundra and still contained pockets of melting ice. Of special interest at that period was the fact that the coast of New England extended further eastward than the present, so that trees and other terrestrial organisms grew in areas that are now submerged beneath the saltwater of the Gulf of Maine. For the last 9000 years, New England has been carpeted by a shifting mosaic of forest vegetation and wetlands. Recent forest history in New England since colonial settlement has been influenced by a variety of natural forces (e.g., windstorms, fire, insect outbreaks, and disease) and human activities (e.g., farming, farmland abandonment, logging, and urbanization). Most of the present forest cover at lower elevations in New England has originated from old-field succession on abandoned farmland or from regrowth of cutover forested lands.

7.3 Discovering Secrets of the Forest When you enter a forest stand, it usually does not hum with obvious activity or indications of vigorous ecological interactions. Yet, the fabric of the forest community is continually being shaped and altered by the relentless interactions among plants, mammals, birds, insects, and microbes involving processes of competition, predation, herbivory, disease, symbiotic relationships, and other forms of interference or influence. How can we open our eyes to see evidence that reveals the life and secrets of the forest? Examples of clues we might notice include deer droppings and rabbit pellets, cone scales left by a red squirrel, fresh leaf litter, pileated woodpecker cavities in a tree, leaves with disease or holes from insect attack, dead tree stems, an old stone wall, stumps that have been cut, fire scars, toppled trees from a wind-throw event, variations in edaphic conditions, presence/absence/abundance of understory plants, epiphytes growing on trees, and perhaps what you do not see – such as missing seedlings that have previously been grazed by mammals. In the understory of a northern hardwood forest, the dead stems of pin cherry saplings may imply that these “intolerant” trees have been unable to compete successfully with taller over-story trees for light, water, or nutrients. In the forest canopy above, the sounds of male birds engaged in territorial defense indicate that competition for breeding sites is another important biotic interaction within this system. At ground level, disease processes may be evident in the cankered stems of beech trees that have been attacked by a beech scale insect that transmits a fungal pathogen. Mushrooms near the base of trees are likely evidence that the tree is a host for a symbiotic mycorrhizal fungus. Finally, the scattered remains of conifer cone scales and seeds and the partially eaten leaves of understory plants are evidence of the ongoing presence of herbivory in the forest. Based on the photographs in Fig. 7.1, consider how you would describe ecological features in the four different forest stands from the perspective of a plant

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Fig. 7.1  Old growth (a – upper left), disturbed conifer stand (b – upper right), white pine regeneration (c – lower left), and disturbed birch stand (d – lower right)

ecologist or an avian ecologist. Is there evidence that tells you something about ecological conditions? In Fig. 7.1a, we clearly have an old-growth forest with very large trees and notice that the base of the tree on the right has a black fire scar that suggests that fire may be an important environmental factor in this stand (and the thick bark of the huge trees may help to protect them from this disturbance factor). In Fig. 7.1b, we see evidence of tree mortality, with downed stems that may be the result of disease processes or wind damage. In the same stand, it is striking to see the relatively bare ground layer, which may imply that overstory canopy foliage is so dense that it shades the ground and prevents the growth of most seedlings. In Fig. 7.1c, there is a dense patch of young white pine that may be taking advantage of a disturbance gap in the forest where light is plentiful for new growth. Finally, in Fig. 7.1d, we see decaying stems of white or paper birch on the forest floor that indicate the occurrence of an earlier mortality event – in this case, it was a major ice storm that snapped the tops off paper birches, setting the stage for changes in the forest community.

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7.4 Using Metrics to Quantify Forest Conditions With our senses sharpened and on alert, the next step is to identify a set of metrics or measurements that can be used to distinguish and to quantify the ecological characteristics of a forest stand. Consider how you would describe the ecological features in a forest community from the perspective of a plant ecologist, an avian ecologist, or a food web ecologist. A reasonable starting point is to determine the richness and diversity of plant species – how many different species occur in the canopy and understory of the forest and are they common or scarce? Next, one might conduct a demographic survey to characterize the densities, sizes (height and diameter), and age structure of the tree species, including the proportions of live and dead stems for each species. These data could be synthesized into graphical results such as those in Figs. 7.2 and 7.3 that provide visual illustrations of stand attributes. In the case of Fig. 7.2, we see that American beech is the most abundant tree and is mostly represented by live stems, whereas aspen is much less abundant, with >50% dead stems. The next graph illustrates that the average diameter of the mature American beech trees is roughly twice as large as the mean diameter of paper birch trees in the stand (Fig. 7.3). As such, these figures help to communicate distinguishing features of the different species that contribute to stand composition and structure. One of the formal tools that is used to quantify and to compare species diversity is the Shannon-Weiner Diversity Index, which accounts for richness (number of species) and evenness (relative abundances of species) in forest communities or other systems of interest. If a forest community has a relatively large number of species and relatively similar densities for each species, the diversity index will be higher, whereas a community with lower richness or a more skewed (uneven) distribution of individuals among species will tend to have a lower diversity index. The Shannon-Weiner Diversity Index (H′) is computed as follows:

Fig. 7.2  Forest stand data showing densities of live and dead stems by species

98 Fig. 7.3  Mean diameter data for a forest stand

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Yellow birch American birch Paper birch Red maple 0

10

20 30 Diameter (cm)

40

Fig. 7.4  Species rank abundance curve (also termed a dominance-diversity plot)



H     pi log e pi  for i  1to s 



Where pi = proportion of ith species and s = # species. The numerical information content of a Shannon-Weiner Diversity Index can also be applied in graphical form using a Species Rank Abundance Curve (also termed a dominance-diversity plot). In this approach, species are plotted left to right from most abundant to least abundant, which results in curves that are steeper or flatter (reflecting evenness), with richness depicted by the final point along the x-axis (Fig. 7.4). In the example, the number of species in A and B is similar, but B (with a steeper curve) is dominated by a few abundant species; thus, community A (with a flatter or less sloping curve) has a more even numerical distribution and would be expected to have a higher Shannon-Weiner Diversity Index value. Suppose we wanted to be able to describe a forest stand in terms of its dominant species. Would you rank species based on their densities, sizes, or some other indicator? Although there is no single answer to this question, ecologists have generally tried to combine multiple measurements into an integrated metric termed an

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importance value (IV). The importance value is an index that combines estimates of density and basal area (tree cross-sectional area) to determine how species compare in terms of the proportion of total density and basal area represented by each species. Since one species might have a lot of small stems, whereas another species might have a few enormous stems, it is useful to have a way to combine both measures into a relative index illustrating the importance or dominance of each tree species. To compute IV, you need to calculate relative density (RD) and relative basal area (RBA), and then combine the scores to get an importance value (IV) for each species as illustrated in Table 7.1. Given the importance and variability of environmental conditions among forest ecosystems, ecologists will often draw on a diverse toolbox of metrics to identify meaningful site characteristics. A useful starting point is to examine edaphic or growing conditions influenced by differences in soil types and soil physical-­ chemical properties (e.g., texture, moisture, drainage, soil depth, and fertility). Light conditions and slope aspect (compass exposure) are also important variables, along with parameters such as length of the growing season, annual precipitation, and temperature regime. Depending on the study focus, there are many other environmental metrics that may be employed. Every forest stand can be classified in terms of successional status (e.g., early, mid-, or late stages of successional development), signs of disturbance (e.g., logging, fire, windstorm, canopy light gaps) and evidence of stand history (e.g., stone walls from earlier farming). At one extreme, you may encounter a forest that was Table 7.1  Calculation of importance values Step 1 To calculate importance value, you follow a stepwise progression beginning with tree basal area. Take the mean diameter of live trees for each species, convert that mean diameter in cm to meters (divide by 100), convert the diameter to radius (divide by 2), and then use the relationship area = 3.14 × r2 to get the mean basal area for that species in m2 per tree. Next, multiply that mean basal area by the density of live trees for that species (stems/ha) to estimate the basal area per hectare (m2/ha) for that species. After you repeat that for each species, you can sum all the species to get a total live basal area for all trees (m2 of basal area per ha). This is the denominator when you calculate relative basal area. Step 2 Relative density: RD for species A = (density of species A/total live density for all trees) × 100 RD for species B = (density of species B/total live density for all trees) × 100 Repeat for all species Step 3 Relative basal area RBA for species A = (basal area of species A/total live basal area for all trees) × 100 RBA for species B = (basal area of species B/total live basal area for all trees) × 100 Repeat for all species Step 4 Importance value IV for species A = RD + RBA for species A IV for species B = RD + RBA for species B Repeat for all species

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100 Table 7.2  An example of forest stand metrics Parameter Live tree density (stems ha−1) Dead tree density (stems ha−1) Live tree basal area (m−2 ha−1) Dead tree basal area (m−2 ha−1) Living biomass (Mt ha−1) Dominant species  American beech  Red spruce  Sugar maple  Yellow birch

Watershed A 1400 190 22.4 7.0 154

Watershed B 1700 560 29.9 7.8 199

Watershed C 1500 250 21.6 4.1 143

x x x

x x x

x x x

recently burned and contains the vigorous new growth of fire-dependent species such aspen, whereas another forest ecosystem may be dominated by even-aged old-­ growth trees that have been relatively free of disturbance for centuries. With a focus on succession, disturbance, or stress in a forest ecosystem, an ecologist may choose to include metrics to quantify dead stems associated with mortality, to enumerate clues related to disturbance (e.g., cut stumps, fire scars, or wind-thrown trees), or to survey the incidence of disease symptoms or damage from defoliating insects. Likewise, in studies that emphasize possible future trajectories of succession, an ecological investigator may quantify the recruitment of seedlings and saplings that represent propagules for the next generation. Once the different measurements have been assembled, the field data can be used to (i) characterize current conditions in a forest ecosystem, (ii) compare the study site with other forest ecosystems, and (iii) provide a baseline for future comparisons as the forest matures and changes over time. The data in Table  7.2 illustrate the kinds of information and insights that can be generated when a suite of suitable metrics is applied to the analysis of a forest ecosystem.

7.5 Consumers in a Forest Community The plant community in a forest ecosystem provides habitat and supports ecological niches for a rich species pool of consumer organisms. If you enter a forest at dawn or dusk and listen, you are reminded that the biotic community of a forest includes a variety of birds, insects, mammals, soil invertebrates, fungi, and bacteria that are linked in a food web. Generally, we expect a positive relationship between the diversity of consumer species in a forest and two major variables – plant species diversity and habitat diversity. The photos in Fig. 7.5 provide a striking illustration of this concept. The left panel shows a monoculture of mature European beech trees that offers low species diversity and very little vertical structure or heterogeneity from the forest floor to the upper canopy. In contrast, the right panel shows a forest with

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Fig. 7.5  Vertical structure and habitat diversity of a beech monoculture versus a mixed forest stand

a mix of species, age classes, and dense vertical structure in the form of plant growth forms and branching patterns, all of which contribute to habitat complexity for consumer organisms seeking their own particular food resources, nesting opportunities, and living conditions. Serving as one primary dimension of habitat diversity, the vertical structure in a forest can be viewed as a series of horizontal layers proceeding from a forest floor or ground layer that may contain seedlings and herbaceous plants, to an understory with shrubs and saplings of differing heights, and up into the sub-canopy and canopy comprised of mature trees of varying heights and ages. One of the ways in which bird species differentiate themselves within the habitat space and vertical structure of a forest community is through differences in feeding or foraging guilds. A foraging guild is a group of species that prefers certain food items and feeding locations in the vertical structure of a forest. Four major foraging guilds are typically recognized, including ground foragers such as thrushes and robins, foliage gleaners (e.g., chickadees and warblers), bark and branch foragers (e.g., nuthatches and woodpeckers), and aerial foragers such as peewees. In a study of niche differentiation among bird species, Robert McArthur reported that the Cape May warbler preferred to forage on new needles and buds in the upper canopy of conifers, the bay breasted warbler foraged primarily on old needles and branches in the mid-level of the canopy, and the yellow-rumped warbler tended to forage on the lower portion of trunks and branches in conifer forests of New England. Thus, even in a bird genus of closely related warblers, there is amazing divergence in the locations and foraging substrates of three species that feed in the same conifer forests. Herbivorous or phytophagous insects (e.g., caterpillars, leaf miners, aphids, and grasshoppers) are one of the important groups of consumers in forest food webs.

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Although many of these insect species occur at low to intermediate levels on a year-­ to-­year basis in forest habitats, some species oscillate or erupt periodically from low densities to high outbreak population levels. There are at least 18 species of forest Lepidoptera (moths and butterflies) that reach huge outbreak densities every 8–10 years in the North Temperate Zone, often causing severe defoliation of trees and other plants. Some of the most common defoliating insects in North America are the gypsy moth of the eastern deciduous forests, the spruce budworm of the northern spruce-fir zone, and the spruce bark beetle of western spruce-fir-pine forests. Insect densities and population outbreaks are controlled by complex interactions involving (i) plant stress, nutrition, and defensive chemistry; (ii) insect predation, parasitism, and disease incidence; and (iii) environmental conditions (e.g., drought). At lower levels of herbivory on plants, insects may actually increase forest productivity, whereas intense insect outbreaks may cause widespread plant mortality that can change the composition and structure of a forest community.

7.6 Avian Ecology in a Forest Ecosystem The secrets of a forest bird community are not easily revealed, because birds are generally difficult to observe in a dense forest canopy and because there are relatively few studies that have monitored bird populations over long periods of time. This presents a challenge – if you characterize a bird community at one point in time, can you be confident that what you observe in a given year is truly representative of the resident breeding or over-wintering bird community for that forest ecosystem. To shed light on this issue, we can examine data from a multi-year study in New Hampshire and ask whether the bird community in a northern hardwood forest ecosystem can be adequately described with a single summer of sampling. Using field observations of birds collected over 15 years in a northern hardwood forest in central New Hampshire, Richard Holmes and his students tested the null hypothesis that the summer breeding bird community in this forest type is a stable assemblage of species. To their surprise, just the opposite was true. During the 15 year period, three major patterns became evident: (i) diversity of breeding bird species varied from a low of 17 to a peak of 28 species (with a mean of 24 species) in 10-hectare plot; (ii) density of all breeding individuals ranged widely from a minimum of 89 to a maximum of 214 birds; and (iii) populations of the different species fluctuated largely independently of one another. As shown with a time sequence graph in Fig. 7.6, five representative bird species exhibited wide swings in abundance and presence over the sampling period. At one point, the least flycatcher was the most abundant of the five species, but 10 years later, the species was absent from the summer breeding population. Overall, results indicated that avian communities can be very dynamic and can include a mix of relatively stable, variable, or even declining species populations. Results also raised important questions regarding the factors that control the community ecology of birds and other species. In this

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Fig. 7.6  Population trends for 5 bird species in a northern hardwood forest. (Data from Holmes et al. 1986)

study, the investigators suggested that year-to-year variations in bird diversity and abundances were potentially caused by a combination of factors, including: changes in food abundances (e.g., irregular outbreaks of defoliating Lepidoptera caterpillars); occasional instances of harsh late spring and early summer weather; winter mortality losses prior to spring migration; changes in habitat structure from forest succession over the 15 year time window; and ongoing inter-specific competition. Another possible contributor would be the differential effects of nest predators on bird densities. In the final analysis, these results suggest that forest breeding bird communities may exhibit considerable inter-annual variations and that other consumers in the food web may likewise be subject to dynamic fluctuations in populations densities as a function of changing biotic and abiotic conditions. The implication is that our understanding of forest communities and food webs requires a long-term investment of research effort to overcome the uncertainties associated with short-term sampling of organisms that are often highly variable. As a final perspective on forest bird communities, let us examine results from a study that focused on the effects of landscape fragmentation on breeding bird survival and nesting success. Fragmentation refers to the impacts of road construction and human development activities on intact forest cover. Where it occurs, fragmentation interrupts the continuity of a forest and cuts expansive forested land into pieces or fragments that may not provide suitable habitat for forest-dwelling birds. The authors of this study monitored nest success of five songbird species – wood thrush, veery, red-eyed vireo, rose-breasted grosbeak, and ovenbird – in forest plots

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that ranged from (i) small fragments up to 30 ha in size, to (ii) large fragments up to 2350 ha, and on up to (iii) continuous forest cover. Observations indicated that bird reproductive success (i.e., production of surviving young) was below replacement levels in small fragments with core breeding areas 25% of vegetables. When chemical companies developed new products to control pests and diseases in the mid-twentieth century, the farming industry was only too happy to incorporate these synthetic chemicals into production plans. We have now reached the point where annual worldwide pesticide use is over 5 billion pounds, and the United States accounts for 20–25% of this total. What is ironic and unfortunate is that crop losses have continued into the present, pesticide resistance is an ever-growing problem, and there are important environmental and health impacts associated with the use of pesticides. The top three categories of chemical pesticides used in modern agroecosystems are herbicides at 40% of the total, insecticides (17% of total), and fungicides (10% of total). Insect pests have been targeted with the following major classes of insecticides: organophosphates, which disrupt acetylcholine and nervous system

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function; carbamates, which act as a nervous system disruptor; toxic organochlorine insecticides such as dichlorodiphenyltrichloroethane (DDT) and chlordane; and synthetic pyrethroid pesticides, which inhibit sodium-gated channels in nerves and muscles. In addition, biopesticides such as Bt (Bacillus thuringiensis) and pheromones have been used to control insect pests. Agricultural weeds and undesirable herbaceous plants have been targeted with herbicides such as phenoxy and benzoic acid compounds, which act as synthetic growth hormones; dinitroaniline compounds, which inhibit cell division in roots; and glyphosate (Roundup), which has been widely used to control weedy competitors in GM crops despite reports of carcinogenic effects in humans. During a 20-year period from 1994 to 2014, the global use of glyphosate herbicide is estimated to have increased from 56 to 826 million kg. Crop losses from insect pests can potentially be exacerbated by interactions with global warming. It has been reported that global yield losses of rice, maize, and wheat to insect pests are projected to increase by 10–25% per degree of global mean surface warming (Deutsch et al. 2018). This is likely to be most acute where warming causes increases in both insect population growth and metabolic rates. A synergy such as this between insect pests and global warming could put enormous pressure on food production! Given the central role of honey bees in agriculture, we should note that there is growing evidence of global pollinator declines linked to neonicotinoid pesticides, neuroactive compounds similar to nicotine. Mitchell et al. (2017) reported that neonicotinoid pesticides were detected in 75% of 198 honey samples collected from around the world, which emphasizes the widespread potential threat to honey bees and other pollinators from this ubiquitous class of pesticides.

12.2.4 Soil Degradation There is a limit to soil resistance and resilience. One issue of concern in modern agriculture is the threat of soil degradation associated with farmland cultivation. When soil is plowed and disturbed, there are two major outcomes that are almost unavoidable – (i) the topsoil is exposed to erosion by wind and water, and (ii) soil organic matter is oxidized at an accelerated rate, which depletes this precious resource. On average, the annual rate of natural soil formation through weathering and pedogenesis is estimated to be 1 ton per hectare. Yet annual soil erosion in the United States is roughly 18 tons per hectare and is as much as twice that rate in parts of Asia, Africa, and South America. In other words, cultivated lands worldwide are suffering unsustainable losses of their natural soil capital accumulated over centuries and millennia.

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12.2.5 Loss of Plant Genetic Diversity Loss of plant genetic diversity is another significant concern. In agroecosystems, modern breeding and selection processes have optimized crop performance at the expense of declining genetic diversity. At risk is the long-term resilience of cropping systems that are based on a limited pool of genetic variation and are thus more susceptible to unexpected biotic or abiotic threats. In an ideal sustainable scenario, the farming industry would maintain a large gene pool of wild seeds and plant phenotypes that could provide a genetic buffer for mitigating future threats to current specialized cultivars. One example of the erosion of genetic diversity in crops can be seen with the common soybean (Hyten et al. 2006). Evidence indicates that the cultivated soybean (Glycine max) was domesticated from its annual wild relative (Glycine soja) in China about 5000 years ago. Since that period, soybean has experienced several genetic bottlenecks resulting from (i) the domestication of selected cultivars in Asia, (ii) the export and introduction of a limited subset of Asian landraces to North America, and (iii) subsequent selective breeding over the last 75 years. Hyten et al. (2006) concluded that the bottleneck with the greatest impact was domestication, which appears to have eliminated the majority of rare alleles. Soybean genetic diversity has suffered further erosion by human selection after domestication. Modern cultivars contain 72% of the sequence diversity present in the original Asian landraces and have lost 79% of the rare alleles found in the Asian landraces. As such, we can ask how effectively these more specialized cultivars, with diminished genetic variation and reduced population-level resistance and resilience, might respond to changing climate and other stress factors. We know that natural selection can be harsh, but a crop that maintains sufficient genetic variation has an improved likelihood of persisting, even when sensitive genotypes are eliminated.

12.2.6 Genetic Modification of Crop Plants Against a background of modern selective breeding, we have now entered an era of biotechnology in which genetically modified (GM) crops are becoming an important part of the human food chain. Global acreage of GM crops has increased to over 200 million acres worldwide. Potential benefits of GM crops have been reported to include increased crop yields; increased plant tolerance to cold, drought, and salinity; reduced costs for food or drug production; increased resistance to pests and disease; reduced need for pesticides (e.g., with Bt gene); enhanced nutrient composition and food quality; and medical benefits for our growing world population. A question many have asked is: are there ecological or health trade-offs associated with these crop plants? Two of the dominant strategies in GM crops are (i) providing a crop with its own biopesticide (Bt) to ward off insect attack and (ii) providing a crop with an internal

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defense against the herbicide glyphosate so that surrounding weeds can be eliminated by herbicide applications without harming the target crop. As one example, GM Bt corn plants exhibit resistance to European corn borer through the expression of insecticidal protein Cry1Ab from Bacillus thuringiensis. Similarly, herbicide tolerance in GM soybeans is conferred by the expression of a plant enzyme isolated from the soil bacterium Agrobacterium tumefaciens. Current major GM crops include corn, soybean, sugar beet, canola, and cotton. One issue that has been raised regarding the risks and unknowns of GM crops is that consumers may be exposed to new allergens – much like peanut allergies – in these novel genetic variants. Scientists have also been concerned that insects will become resistant to GM plants containing the Bt gene so that the advantage of GM plant self-protection will be essentially lost. In fact, Monsanto revealed in 2010 that a common insect pest had developed resistance to its GM cotton, which contains the Cry1Ac gene from the bacterium Bt; other similar reports have followed. Another problematic concern is that the horizontal gene transfer of pesticide or herbicide resistance to other organisms could allow non-target plants to grow in an uncontrolled fashion. Finally, some scientists have expressed reservations regarding the unknown outcomes of vertical gene transfer between GM plants and their wild-type counterparts. In an effort to address the many uncertainties regarding GM crops, the National Academy of Sciences (NAS) (2016) issued a detailed critical review entitled Genetically Engineered Crops: Experiences and Prospects. In this report, top university scientists were recruited to prepare a comprehensive review of evidence focused on the agronomic, economic, health, social, and environmental effects of GM crops. The authors provided a number of conclusions. In locations where resistance management strategies were not followed, damaging levels of resistance to Bt evolved in some target insects. In areas where the planting of herbicide-resistant crops led to heavy reliance on glyphosate, some weeds evolved resistance and now represent a major agronomic problem. Besides the issue of evolving resistance in pest species, the committee generally found no definitive evidence of cause-and-­ effect relationships between GM crops and environmental problems. Regarding health impacts, the authors reported that although the design and analysis of many animal feeding experiments were not optimal, the large number of experimental studies provided reasonable evidence that animals were not harmed by eating food derived from genetically engineered (GE) crops. Additionally, long-­ term data on livestock health before and after the introduction of GE crops showed no adverse effects from GE crops. The committee also examined epidemiological data on the incidence of cancers and other human health problems over time and found no substantiated evidence that foods from GE crops were less safe than foods from non-GE crops. Furthermore, the committee did not observe any relationship between the consumption of GE foods and the increase in the prevalence of food allergies. The NAS committee concluded that no differences have been found that indicate a higher risk to human health safety from GE foods than from their non-GE

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counterparts. However, the authors noted that they cannot rule out subtle adverse health effects that have not yet been detected or may develop over longer time periods. One issue that cannot be ignored is the concern about the potential carcinogenicity of glyphosate in humans. While there is disagreement among the World Health Organization, the European Food Safety Authority, Canada’s health agency, and the US Environmental Protection Agency (EPA) on the potential health harm that could result from the use of glyphosate on GE crops, recent legal cases indicate that the expanded use of glyphosate on GM crops may present a nontrivial health risk to farm workers and perhaps consumers.

12.2.7 A Recap of Concerns We can summarize the major concerns and risks that confront us in modern food production as follows. (i) Can food production keep up with the population? (ii) Crop losses are high, so how do we control pests? (iii) We depend on irrigation, but water supplies are suffering – what is a sustainable solution? (iv) Nonpoint pollution is a growing problem resulting from overuse of fertilizer – can we get this under control? (v) Soil degradation is making it harder to increase yields – how do we restore and maintain soils? (vi) Energy inputs in agriculture are too high, and genetic diversity is too low – are we willing and capable of making the necessary changes? Given these serious concerns with modern industrial farming, what alternative approaches are being tested?

12.3 Sustainable Agriculture – An Ecological Vision for Food Production Sustainable agriculture is an approach that applies ecological principles and organic farming techniques to crop production. A sustainable farming operation is one that intentionally and systematically focuses on the following kinds of goals: • • • • • • • • •

Reduce the carbon footprint of food production and distribution. Minimize weeds and pests with low impact approaches. Minimize nutrient runoff and pollution. Minimize pesticide use and toxicity. Sustain soil fertility and soil organic matter and eliminate erosion. Maximize crop yields but with lower inputs of water and nutrients. Minimize transportation expenses and impacts. Diversify food and crop resources. Encourage consumers to decrease meat consumption and to  eat lower on food chains.

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One of the major tools in sustainable agriculture is integrated pest management (IPM), which focuses on minimizing pesticide applications by using an ecological analysis of pest and disease organisms. In this approach, farmers work with scientists to understand the ecology and natural history of crop pests and diseases, and that ecological knowledge is combined with pest and disease surveys that provide an advance warning of potential outbreaks. Biological controls such as sterile males, pest predators, parasitic wasps, and fungal pathogens can be used to reduce the incidence of pest species. Furthermore, rather than applying regular doses of insecticides and fungicides, farmers can wait until field surveys indicate that a pest outbreak is imminent and can then use pesticides judiciously to knock down a pest eruption. Overall, IPM can help shift pest management away from heavy reliance on chemical toxins in favor of more sustainable methods of pest control. Thies and Tscharntke (1999) described an example of managing farm landscape structure to promote the biological control of pollen beetle pest impacts in fields of the oilseed plant Brassica napus. The authors reported that maintaining unmanaged old field margin strips along cultivated oilseed fields resulted in increased parasitism and mortality of pollen beetles. Evidence indicated that in complex landscapes, pollen beetle parasitism was higher and crop damage was lower than in simplified landscapes containing a high percentage of agriculture. They concluded that a prudent IPM approach would maintain natural habitat around and between fields as a low-impact way to maintain local populations of natural predators and parasites that decrease pest damage with minimal collateral damage. To generalize their results, we can say that one important aspect of IPM is the management of land cover – in other words, farmers should (i) pay attention to landscape structure and avoid expansive monocultures that attract pests and (ii) promote heterogeneous landscape conditions to support natural populations of biological controls. Weeds are relentless, so what can be done if you want to grow crops sustainably with minimal inputs of herbicides? By some means, it is essential that the balance be tipped in favor of crop plants by limiting the opportunities for colonization and exploitation of resources by weed species. Because weeds need space to grow, farmers can interplant complementary crop species, such as corn and soybeans, that cover much of the soil surface, leaving limited space for competitive weeds. Furthermore, farmers can utilize flexible timing of planting, cultivation, irrigation, and fertilization in order to give crop plants advantages over weed competitors. By using field rotation with cover crops, farmers can improve soil quality while reducing the buildup of weed seed banks that might accumulate in the soil of fields that are cultivated year after year for a single monoculture crop. Finally, farmers can rely more heavily on physical weeding and mulching as a means of reducing weed competition and preventing weeds from shedding seeds that reside in the soil awaiting the next opportunity for germination. Farming is in some respect an extractive practice – in other words, plants absorb nutrients from the soil, and the plants are then taken away to be eaten elsewhere. Thus, nutrients are extracted and removed from the soil. In the absence of nutrient replenishment from mineral weathering, microbial processes, atmospheric inputs,

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or farming practices, the soil nutrient capital is gradually depleted. If at the same time soil organic matter declines as a result of cultivation or a lack of regular inputs of detritus, soils can become less and less able to support crop growth. One of the challenges of sustainable agriculture is to maintain the fertility and natural capital (i.e., minerals and organic matter) of the soil using low-impact environmentally sound principles. Cover crops, green manures, and low-tillage techniques provide a way to improve soil organic matter and decrease erosional losses. Crop rotations with fallow fields can be used to allow soils to recover from intensive extractive production. Nutrient management techniques can also be used to minimize nonpoint pollution and greenhouse gas production; these objectives may be accomplished with either low-tech methods or with GIS and computer-assisted fertilizer applications. As one example of a low-budget approach, Matson et  al. (1998) reported that lower doses of fertilizer applied later in a crop cycle in Mexican wheat fields reduced gaseous losses of the greenhouse gases N2O and NO and generated increased profits for farmers. Overall, study results demonstrated that an economical knowledge-intensive approach to sustainable farming could reduce both environmental impacts and farmer costs. As a final issue regarding sustainable agriculture, let us take a look at corn-based ethanol production for energy use. This is an interesting topic with a number of different dimensions and interactions. First off, a number of studies have concluded that the energy yield for ethanol derived from corn is less than the life cycle energy input to produce the ethanol, which clearly implies an unsustainable outcome. Adding to the body of evidence concerning farm-to-factory ethanol is the fact that corn biomass that is diverted into ethanol competes with the availability of corn for food; furthermore, new nonfood demand drives up the price of corn for consumers. Ultimately, food availability decreases, and world hunger increases. In addition, ethanol demand may promote expanded acreage in corn cultivation, which generates more nonpoint pollution from nitrogen fertilizer runoff – this threatens aquatic sustainability. Fortunately, the focus is now shifting toward cellulosic ethanol as an energy fuel, which frees up corn for human consumption and provides a number of benefits in terms of sustainability. As an example, Davis et al. (2012) examined the benefits of transitioning from corn-based to switchgrass cellulosic ethanol. Their analysis indicated that ethanol production from switchgrass yields 82% more ethanol, reduces nitrogen leaching by 15–22%, and reduces greenhouse gas emissions by 29–473%. The authors concluded that large-scale conversion from high-input annual corn crops to low-input perennial crops of switchgrass can transition the central United States from a net source to a net sink for the greenhouse gas carbon dioxide (CO2).

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12.4 Another Issue – Use of Antibiotics in Agriculture We are all well aware of evidence that pathogenic bacteria that cause human diseases have developed resistance to many common antibiotics (Witte 1998). The evolution of resistance to antibiotics is facilitated by the overuse of antibiotics in ways that allow them to act as selective agents. Unfortunately, it is now apparent that widespread prophylactic use of antibiotics in animal feeds has exacerbated this problem and has accelerated the development of resistance in pathogens that affect humans. One example of the disproportionate use of antibiotics in animal husbandry comes from Denmark. A report from 1994 indicated that 24  kg of the antibiotic vancomycin were used for human therapy in Denmark, compared to 24,000 kg of the similar antibiotic avoparcin used in animal feed during that same year. The message from these kinds of data is that there is an urgent need to limit routine antibiotic use in cattle, pig, and poultry production. Why would we want to encourage food production methods that drastically raise the odds of human suffering from persistent antibiotic-resistant pathogenic microbes?

12.5 Looking into the Future Agriculture is a huge potential driver of change in the biosphere because so many resources are used to grow and transport the enormous quantities of food consumed by the eight billion humans on the Earth. In this short chapter, we have seen the challenges that confront us in modern farm production systems and have hopefully come to appreciate how vitally important it is to reduce the ecological footprint of agriculture and to seek ever-more sustainable solutions in our farming practices.

Chapter 13

Ecological Models

When I arrived at graduate school, my advisor decided to make life interesting by asking me to develop a computer program to predict the size of a deer population over time so that he could use it as a tool in his ecology class. What I soon discovered was that the task was a bit like peeling an onion with many layers. At its core, my mathematical program required the ability to predict population size as a function of births, deaths, immigration, and emigration over time. Yet each of those parameters was affected by a mix of ecological factors such as food availability, winter snowpack severity, predator density and kill rates, population age structure, effects of environmental conditions on fertility rates, disease incidence, habitat conditions, and hunting mortality rates. After many iterations and rounds of problem-­ solving, I was eventually able to produce a computer model that combined ecological data and mathematical relationships to predict the deer population size on a monthly time step over a multiyear period. But that was just the beginning – the next step was to test the model against real-world data. How about your own experience  – have you ever encountered mathematical models in your reading or coursework? My guess is that you have. For example, during the COVID-19 pandemic (circa 2020), daily news reports were filled with projections from epidemiological models describing likely future trends for the rates of infection and mortality from the coronavirus SARS-2. Likewise, modern weather forecasts on the nightly news depend on meteorological models, and other types of models are widely used across a range of scientific disciplines, from environmental science to medicine and public health. For example, complex general circulation models (GCMs) are used to integrate biosphere dynamics and feedbacks in land-ocean-atmosphere systems in order to simulate future potential changes in climate. This chapter is intended to make us aware of the ways in which models can provide valuable perspectives, insights, and understanding in the analysis of ecological problems. Models in ecology vary widely in form and function. In some cases, ecological models are developed primarily as qualitative conceptual frameworks that serve as © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. S. Cronan, Ecology and Ecosystems Analysis, https://doi.org/10.1007/978-3-031-45259-8_13

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tools for organizing what is known about a problem. More often, however, a conceptual model is just the first step in the design and development of a quantitative computer-based model that combines data and insights from ecological analysis and mathematics to produce a numerical representation of an ecological process, pattern, or system. Some models are relatively small and simple (e.g., the logistic model for the population growth of one species), as contrasted with others that are large and complex, such as ecosystem models or global climate models. One of the more unusual examples of a modeling approach is the agent-based model that incorporates so-called cellular automata. The first generation of these models focused on simulating social behavior in an ant colony. Each ant was represented as an “agent,” and the objective was to encourage individual ants to behave in a socially cohesive fashion as an integrated colony. Much to their surprise, modelers found that if ants followed a set of roughly six rules of behavior, their collective individual actions would produce “emergent” group behavior of the sort expected in an ant colony. Who would have guessed that six simple rules could produce order from chaos? My faculty colleague later applied this modeling approach to simulate the behavior of humans in a Maine lobster fishery (Wilson et al. 2007).

13.1 Modeling Goals and Objectives Before we proceed any further, let us ask the question: “why would I be interested in learning about ecological models,” especially if I am not very fond of mathematics? My answer to that is that models represent a powerful learning tool that can be used by almost any scientist, with or without a high math IQ. In fact, many models are the result of collaborations involving ecologists or biologists working with colleagues who contribute expertise in computer programming or math to the development of a model. Returning to the question of “why models,” there are at least three fundamental reasons why models are important: (i) they force us to define how much we understand about a system or process, (ii) they help identify information gaps for the topic of interest, and (iii) they provide a quantitative tool for integrating knowledge and generating testable hypotheses and predictions. Models are most effective when they are used as a synergistic tool in conjunction with experimental or empirical research. In this interactive role, models allow us to: (i) Compile and integrate information and current understanding (ii) Identify gaps in our knowledge base and current understanding (iii) Explore feedbacks and interactions among multiple variables in a study system (iv) Generate questions and hypotheses for empirical testing (v) Evaluate the apparent sensitivity of ecological relationships and variables, and (vi) Make predictions and simulate outcomes of new scenarios (with due caution)

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13.2 Considerations in the Development of a Model Suppose we wished to design a numerical model to simulate nitrogen (N) cycling processes in a forested watershed. How would we organize a strategy to accomplish this task? To begin, it would be important to build a conceptual framework based on a set of modeling objectives and a list of ecological questions that would be addressed by the model. Next, it would be necessary to decide what assumptions would be incorporated into the model in terms of key variables, processes, interactions, and simulation time steps. For example, would it be acceptable to assume that all woody plants resorb foliar nitrogen at the same rate? It would also be important to determine the spatial and temporal scales and the degree of aggregation that would be incorporated into the model and to assess how these choices would affect resolution, accuracy, and efficiency. How would system components or processes be represented in terms of a balance between simplicity and realism? For instance, would we select a generic type of tree for our forest, or would the model include a realistic variety of tree species that might differ in nitrogen cycling characteristics? It would then be necessary to determine how these choices would affect the time required to build the model, computer run times, data requirements, and applicability of the model to other systems or sites.

13.3 Steps in Building a Model We can visualize the steps in building a model by referring to the diagram in Fig. 13.1. In the first step, a conceptual model is produced that depicts the overall network of compartments or state variables and the processes or flows connecting them into an integrated system. As an example, Fig. 13.2 illustrates a conceptual framework that could be used to develop a quantitative model for carbon cycling in a forest ecosystem. Using the conceptual diagram as a road map, a modeler would take each of the components and assemble empirical data that could be used to quantify relationships (step 2) necessary to produce the computer code for a numerical model of carbon cycling (step 3). After a model is fully developed, a modeler uses the best available data to calibrate or “tweak” the computer code so that it can

Fig. 13.1  Steps in producing a computer simulation model

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Fig. 13.2  Conceptual model of the compartments and transfers in a forest carbon cycle

Fig. 13.3  Validating a model of stream chemistry by comparing model predictions with an independent empirical data set from the field. (Data from Gbondo-Tugbawa et al. 2002)

simulate real-world ecosystem behavior (step 4). In the final validation stage of model development (step 5), a new independent data set is used to challenge the model to determine whether it can simulate a new novel set of conditions to produce accurate outputs. For example, Fig.  13.3 shows the validation of a stream water chemistry model that was found to underpredict sulfate concentration as compared to actual field observations. In response to this outcome, the modelers were forced

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Fig. 13.4  Comparison of global warming predictions from 12 general circulation models under a scenario with a doubling of atmospheric CO2 concentration. (Data from Cess et al. 1993)

Temperature Increase ( C)

to revise their model structure in an attempt to more accurately simulate the patterns of sulfate concentration in the stream. As such, the validation stage can be a necessary learning step that leads a scientist “back to the drawing board” to figure out where the gap is in terms of understanding the system being modeled. Once the validation stage is successfully completed, a model can be applied to its original research purposes and can be refined through iterative testing (step 6). We should note that the validation of climate change models is very challenging because these models make predictions about an unknown future, which would theoretically require waiting decades to see if the Earth’s climate system behaved as expected. This difficulty has been addressed in two major ways. In a number of instances, independent groups of scientists have agreed to apply their separate models to simulate the same future scenario with the aim of testing whether the results converge in a fashion that would imply some degree of “truth.” In Fig. 13.4, we see a comparison of multiple independent models that were tested to determine whether the models are more or less in agreement regarding the impact of a doubling of atmospheric carbon dioxide (CO2) on the mean global temperature. Results indicated that models predicted anywhere from a 1.7 to a 5.5 °C increment of global warming over the next century, which indicates a rather large range of uncertainty. In other words, this example of a validation strategy suggests that further refinement is warranted on these models. A second option is to build a climate change model and use it to predict a previous time period based on historical data. With this “hindcasting” approach, a model is calibrated with one set of historical data and then is tested for validation using another data set from the past, with the caveat that the historical data are likely to be incomplete with respect to many of the climate parameters required by the model. Unfortunately, neither option is optimal, but that is part of the challenge we face with making predictions about future climate change scenarios.

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13.4 An Example Based on a Model of a Forest Nitrogen Cycle In the section that follows, we will examine a simple nitrogen cycling model to see if maybe we can be surprised at the unexpected or nonintuitive predictions that arise when we incorporate the pieces of our understanding into an integrated system of logic. Our N cycle model links the sinks and sources of nitrogen and determines whether plant demand for N is adequate to prevent N leaching loss to streams. The model was developed as a teaching tool to challenge a prevailing view that atmospheric deposition of nitrogen was fertilizing forests to the point where they became “saturated” with more than enough nitrogen so that excess nitrogen was being exported into streams where it threatened aquatic systems with eutrophication. With this model, we can explore alternative explanations for changes in stream exports of nitrogen from forest ecosystems. In fact, we can ask whether relatively small variations in forest ecosystem parameters (e.g., foliar nitrogen concentration, foliar resorption of nitrogen, or soil decomposition rate) can potentially translate into symptoms of elevated stream N export that might otherwise be ascribed to atmospheric deposition and nitrogen saturation. At its core, our N cycling model tracks the mass balance of annual nitrogen supply minus annual demand for nitrogen by the forest ecosystem in order to determine if there is any excess soluble nitrogen left to exit the watershed in stream runoff. That would be the case if annual N sinks in plants, microbes, and soil are less than annual sources of N supply (as illustrated in the simplified summary in Table 13.1). If we step through the six panels in Tables 13.2a, 13.2b, 13.2c, 13.2d, 13.2e and 13.2f, we can see how different scenarios of change in individual ecosystem parameters generate different outcomes with respect to potential stream exports of excess soluble nitrogen. In the base case depicted in panel A, the model parameters are set to mimic average annual conditions in the forest ecosystem based on values in the published literature for this ecosystem. As can be seen in the base case summary, forest N supply and demand are almost in balance, except for a small net stream export of 4 kg N per hectare per year (see box in lower right corner with red ink). In the next panel B, Scenario 1 simulates a year when there is a decrease in foliar N

Table 13.1 Simplified structure for a forest nitrogen cycling model

Model variable Annual plant demand Above ground Below ground Net soil N mineralization Atmospheric N input Available soil N Supply-demand Potential N export in stream

Kg N ha−1 year−1 50 50 100 8 108 8 8

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Table 13.2a  N cycle model (base case)

Table 13.2b  N cycle model (decrease foliar N to 0.01 or 1%)

concentration, with a decline from 0.02 to 0.01 (i.e., from 2% to 1%). What is your expectation for how this might or might not translate into any impact on stream exports of N? To begin, we can conclude that the decrease in foliar N means that there is less plant demand for nitrogen to grow the annual crop of leaves. If the

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annual N supply rate from atmospheric deposition and decomposition stays the same as in the base case, the decrease in plant nutrient demand should leave additional “unused” soluble N in the soil that can leach into stream runoff under the influence of rainfall. Indeed, that is what the model predicts – cutting the foliar N concentration in half produces a weaker forest sink for nitrogen, and the leaching of that excess soil nitrogen results in more than a fivefold increase in the stream export of soluble N (see the red value of 22 kg ha−1 year−1 inside the box). In Scenario 2 depicted in panel C, the foliar resorption of N is increased to 60% per year as compared to 40% in the base case. Thus, the forest stores 50% more resorbed N for foliage production the next year and requires less uptake of N from the soil. Perhaps you can predict how this will translate into a change in stream chemistry. Again, this makes the plant community a weaker sink for soil nitrogen, which leaves more “unused” soluble soil nitrogen available for leaching into the stream. The end result is a stream export value of 16  kg  N per hectare per year, which is four times higher than the base case. Thus, a relatively small and realistic year-to-year variation in the internal recycling of N in the tree canopy can potentially have a measurable impact on N cycling across the entire watershed, ultimately promoting a substantial uptick in stream export of N. Table 13.2c  N cycle model (increase foliar resorption to 0.6 or 60%)

Scenario 3 in panel D allows us to consider the opposite condition – when annual foliar resorption is only 20% as compared with the base case of 40%. Thinking this through our system of logic, this means that internal plant cycling supplies much less N for new leaf production, which implies that the forest must absorb much

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more soluble N from the soil, making the forest a stronger N sink. The fact that the stream N export box shows a negative number means that forest demand for N is so great that there is nothing left for stream export (in other words, it would actually be 0 rather than the theoretical value of −8 kg N per hectare per year, shown in red ink). Written as it is, the stream export box emphasizes the large swing from Scenario 2 at a value of +16 to Scenario 3 at a value of −8 kg N ha−1 year−1.

Table 13.2d  N cycle model (decrease foliar resorption to 0.2 or 20%)

Forest growth varies from year to year in response to changing weather and environmental stressors, and Scenario 4 in panel E demonstrates how N cycling in a forest ecosystem is sensitive to rates of forest primary production. When net primary productivity (NPP) is decreased from the base case of 10% per year to 5% per year, stream export increases tenfold from the base case (rising from 4 to 44 kg N ha−1 year−1). This is a dramatic illustration of the impact of NPP on a forest ecosystem N sink – here, the forest has become a much weaker sink for N, and there is consequently a lot of unused soluble N in the soil that can leach into the stream.

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Table 13.2e  N cycle model (decrease NPP from 10% to 5% per year)

In the final panel F, Scenario 5 focuses on the fact that forest soil organic matter contains a huge pool of nitrogen that decomposes very slowly but at a rate that is not well understood. The base case assumes that the organic N pool decays at 0.002 per year (0.2% per year), but it could easily be twice that at 0.004 per year. If the higher rate applied and the forest community continued at the same rate of growth without exploiting that extra available nitrogen, there would potentially be almost five times as much stream export of nitrogen as compared to the base case (4 versus 19 kg N ha−1 year−1). Again, we see a multiplier effect whereby a doubling of the rate for one parameter can create conditions for a much larger (fivefold) impact on the stream export of N.

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Table 13.2f  N cycle model (double the soil organic matter decay rate)

To summarize this short modeling exercise, our simple model suggests that there are multiple possible explanations for the symptoms of elevated stream export of nitrogen associated with the phenomenon of “nitrogen saturation.” Yes, atmospheric deposition of excess nitrogen is a problem, but it appears that it is one of several processes that can contribute to higher-than-expected exports of stream N. The N model suggests that realistic variations in multiple nutrient cycling parameters in a forest ecosystem can translate into marked changes in the stream export of N. Without the model, we might have guessed otherwise.

13.5 Applications of Biogeochemical Models The literature contains a fascinating collection of ecological models spanning physiological, population, community, ecosystem, landscape, and global scales of resolution. In the section that follows, selected examples of models are briefly described to illustrate a cross-section of these research efforts.

13.5.1 TREGRO – A Model to Simulate Plant Responses to Interacting Stresses The simulation model TREGRO was developed to analyze the responses of red spruce trees to multiple stresses associated with drought, nutrient deficiency, and pollutant exposure (Weinstein et al. 1991). The intent of the model was to predict

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patterns of growth and carbon allocation for individual trees as a function of growing conditions. During a simulation run, TREGRO monitors plant carbon balance by (i) calculating the photosynthesis of a red spruce tree each hour as a function of environmental conditions and the availability of light, water, and nutrients; (ii) allocating daily redistribution of carbon throughout the plant; and (iii) accounting for the loss of carbon via respiration and senescence. To accomplish this task, the model tracks the flow of CO2 to the sites of fixation at the needles, the availability of light in the canopy, the supply of water and nutrient resources in each of the soil horizons, and the amounts of these resources absorbed by the tree. Water status, photosynthesis, and leaf respiration are simulated every hour, and nutrient uptake, C allocation, and growth are computed on a daily basis. The sequence of events simulated by TREGRO during a daily time step includes the following actions: calculate gross photosynthesis and plant respiration, calculate net carbon available for allocation, grow aboveground tissues, absorb nutrients from the soil, grow root tissues, and store excess carbon. Following its initial development, the TREGRO model was used to investigate the potential effects of elevated atmospheric concentrations of the oxidant ozone on mature sugar maple trees (Retzlaff et al. 1996). After 3 years of simulated exposure to twice ambient ozone concentrations, model results indicated that trees exposed to increased ozone experienced a 14% reduction in carbon gain compared to unexposed trees. This simulated reduction occurred in the nonstructural carbohydrate storage pool of the simulated tree, with the majority of the reduction located in the coarse woody roots. The authors noted that a response such as this might be expected to compromise the competitive ability and stress tolerance of a tree growing in a forest community. An important question we might ask is, how did they validate the model? As it turns out, TREGRO was part of a multi-investigator study that included paired experimental studies with young trees that were exposed to a range of growing conditions and stresses that provided empirical feedback for the modeling effort.

13.5.2 TEM: A Global Model of Net Primary Productivity Raich et al. (1991) designed a mechanistic terrestrial ecosystem simulation model (TEM) that used spatially referenced information on soils, vegetation, and climate to estimate pool sizes and fluxes of carbon and nitrogen, along with monthly primary production at continental to global spatial scales. Their primary objective was to describe the spatial and temporal patterns of net primary productivity in South America as a function of a limited set of environmental variables. The authors combined a highly aggregated and simplified model of plant/soil processes affecting NPP with a grid-cell-based Geographic Information System (GIS) model of environmental conditions and plant production. The model predictions of NPP in each grid cell location were driven by calculations of gross primary productivity (GPP) based on solar energy inputs, CO2 gas concentration, moisture availability, air temperature, nitrogen availability, and relative photosynthetic capacity of a given

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vegetation type on the ground. Monthly estimates of NPP were then computed for each grid cell as GPP minus rough estimates for autotrophic respiration. Predictions from the TEM model were used to produce GIS spatial maps of potential annual NPP for South America and monthly estimates of NPP for major vegetation cover types in South America. The scientists were unable to perform a strict validation of the model; however, they reported that their estimated annual NPP values were generally within the range of values described by other authors for major vegetation types in the region. The TEM estimate of the total potential NPP for non-wetland areas of South America was 26.3 Pg organic matter year−1, compared with a range of 24.4 to 28.8 Pg year−1 reported by Box (1978) for all of South America. As such, the uncertainty for the TEM model predictions is still somewhat unknown and unconstrained.

13.5.3 PnET-BGC – An Integrated Biogeochemical Model The detailed long-term data set generated by investigators at Hubbard Brook Experimental Forest, NH, inspired the development of three successive ecosystem modeling efforts  – PnET, PnET-BGC, and PnET-CN.  In its original formulation, PnET was designed to predict photosynthesis, evapotranspiration, and NPP in forest ecosystems (Aber and Federer 1992). PnET-BGC was later developed by investigators as an integrated biogeochemical model to evaluate the effects of the atmospheric deposition of N and sulfur (S) on forest and aquatic ecosystems (Gbondo-Tugbawa et al. 2001, 2002). A subsequent model, PnET-CN, was used to predict the cycling of carbon, nitrogen, and water in temperate forest ecosystems (Thorn et al. 2015). In PnET-BGC, Gbondo-Tugbawa et al. (2002) included representations of key biogeochemical processes that were assumed to influence watershed cycling and the export of S derived from atmospheric deposition. Atmospheric deposition was estimated from relationships between regional emission patterns and the rates of wet plus dry deposition. Small amounts of internal S release from weathering were included, along with the biotic cycling of S by plants and soil microbes. Finally, a soil chemistry module was incorporated to simulate chemical processes such as sulfate adsorption and soil buffering of acidity. When the authors compared simulated annual volume-weighted concentrations of sulfate in stream water at Hubbard Brook Experimental Forest with measured values for the period 1964 to 1992, it was found that the PnET-BGC model underpredicted actual stream concentrations by an average of 9 μmol L−1. In an effort to account for this discrepancy, the modeling team examined other possible sources of S inputs to the watershed. Their analysis showed that increasing estimated inputs from either dry deposition, internal weathering of a sulfur-bearing mineral, or net mineralization of soil organic S contributed to improved model predictions that were more consistent with observed stream chemistry.

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13.5.4 An Agent-Based Model of Harmful Cyanobacterial Growth Hellweger et al. (2022) developed a mechanistic agent-based model to predict the growth of the harmful cyanobacterium Microcystis in Lake Erie as well as the production of its potent hepatotoxin, a compound known as microcystin. The model was intended to address a water quality risk for drinking water in Toledo, Ohio, and nearby communities that resulted from ongoing lake eutrophication that favored the growth of Microcystis and threatened humans with exposure to dangerous microcystin toxins. Resource managers and scientists from the United States and Canada had decided that the best way to control and to curb this harmful aquatic organism and its toxin was to implement a 40% reduction in phosphorus (P) releases to Lake Erie, with the assumption that this action would limit the growth of all algae, including the undesirable Microcystis. Modelers used a large database derived from empirical studies to establish quantitative relationships in their model and then explored which variables were most influential in determining Microcystis abundance and toxin production by this cyanobacterium species. Based on their modeling results, investigators concluded that the availability of nitrogen (besides P) is a strong control on Microcystis abundance and the overall release of microcystin toxin. Consequently, the scientists recommended that water resource managers revise their plans so as to call for the reduction of both P and N inputs to Lake Erie.

13.6 A Recap of Models as Ecological Tools In concluding this brief overview of models, we can ask again: “why are models of interest and important in ecological analysis?” Perhaps one way to think about an answer is to picture a craft table filled with lots of colorful skeins of yarn. Although each skein has its own merits, it is only when the different yarns are combined and integrated into a sweater pattern that the pieces become a meaningful and beautiful whole. Likewise, models provide a tool for knitting together separate pieces of scientific understanding into an integrated holistic framework capable of providing critical insights regarding ecological patterns, processes, and cause-and-effect relationships in natural systems. Models can thus serve as powerful synergistic learning tools for advancing understanding in both research and educational applications. This is especially true when model development and testing are paired with complementary empirical studies aimed at generating the data required for model calibration, validation, and hypothesis testing.

Chapter 14

Atmospheric Influences, Global Warming, and Climate Change

Humans on planet Earth have quietly and naïvely been conducting a gigantic environmental experiment that has transformed the biosphere. Early on, the changes were either localized or imperceptible. Yet slowly over time, human activities associated with fossil fuel consumption, agriculture, land clearing, petrochemical use, metal smelting, and motorized transportation generated unprecedented stresses in the landscape, the oceans, and the atmosphere. Some of the most subtle but significant impacts of human activities occurred as emissions of airborne substances disrupted the balance of gases and aerosols in the atmospheric envelope. Initially, there was an implicit assumption that the vast atmosphere could absorb these compounds without any ill effects. Sadly, that proved to be false. It is now clear that we have crossed a tipping point marked by serious and undesirable consequences. Changes in atmospheric chemistry stemming from modern anthropogenic emissions present three major challenges: (1) deterioration of air quality, (2) erosion of the protective ozone (O3) layer that limits inputs of damaging UV radiation to the Earth’s surface, and (3) enrichment of greenhouse gases that promote warming, climate change processes, and destabilization of our planet. These atmospheric influences are a major reason why we should be concerned with understanding the basic processes of atmospheric chemistry in an ecology class. The atmosphere is an active and major part of the global environment and is of critical importance because of the ways in which it connects and links virtually all parts of the biosphere. Our discussion of atmospheric influences focuses on the largely invisible class of substances known as anthropogenic air emissions – i.e., gases and airborne particles that originate from human activities. The major emissions of concern are sulfur oxides (SOx), nitrogen oxides (NOx), volatile organic carbon (VOC), black carbon (BC), microscopic particulate matter (PM), carbon dioxide (CO2), methane (CH4), and heavy metals such as mercury (Hg) and lead (Pb), which can occur in volatile forms. Under the influence of multiple chemical transformations and pathways in the atmosphere, these air emissions contribute to five major global environmental © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 C. S. Cronan, Ecology and Ecosystems Analysis, https://doi.org/10.1007/978-3-031-45259-8_14

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stresses: (i) global warming, (ii) acidic deposition, (iii) ozone depletion, (iv) ground-­ level ozone formation, and (v) environmental contamination with airborne toxins. To begin, we can identify the basic characteristics of these different air emissions. The broad category of sulfur oxides (abbreviated as SOx) refers to emissions of sulfur dioxide (SO2), sulfate (SO42−), sulfite (SO32−), and thiosulfates (S2O32−). Sources of SOx are the combustion of coal, oil, and gasoline, along with the smelting of ores containing reduced sulfur. The primary concern with SOx emissions is their contribution to acidic deposition; in addition, sulfur dioxide gas is toxic to plants and many other organisms. NOx is an abbreviation for multiple nitrogen oxides, including nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O), and nitrate ion (NO3−). Mixtures of NOx compounds are formed by the oxidation of atmospheric N2 gas during high-temperature combustion. Major sources of NOx compounds are exhaust emissions from motor vehicles, power plants, and industrial boilers. Nitrogen oxides are remarkable in that they contribute to five major global change issues: acidic deposition, aquatic nonpoint pollution, smog and ground-level ozone formation, stratospheric ozone depletion, and global warming and climate change. Emissions of volatile organic carbon (VOC) or hydrocarbons (HC) include organic vapors and gases such as benzene (from gasoline), chlorofluorocarbons (CFCs) from refrigeration units, and industrial solvents. Natural terpenes vaporized from plant foliage also contribute to VOC concentrations in the atmosphere. VOCs are essential ingredients in smog and ground-level ozone formation, whereas CFCs are active as greenhouse gases and contribute to the depletion of stratospheric ozone. Particulate matter (PM) is composed of fine particles and aerosols originating from various combustion processes. Air quality is degraded by PM, which is a health risk for respiratory diseases and, in some cases, may be carcinogenic. The US Environmental Protection Agency (US EPA) classifies particulate matter into size categories, such as PM-10, PM-5, and PM-2.5, based on particle diameter in microns. One important component of PM is black carbon (BC), which includes the tiny soot particles that form during incomplete combustion of biomass and diesel fuels; thus, BC may originate from both anthropogenic and natural sources (e.g., wildfires). BC is not only a respiratory health risk for humans, but these dark particles are also strong absorbers of solar radiation in the atmosphere and on snow and glaciers, thus influencing global warming processes. Who would guess that human activities could increase the atmospheric circulation of mercury (Hg) and lead (Pb), two very dense heavy metals! As it turns out, both metals can occur in airborne gaseous or vapor phases that allow for widespread distribution from emission sources. Lead largely originates from smelting activities and the historical use of leaded gasoline, whereas mercury primarily originates from coal combustion and waste incineration. After deposition from the atmosphere to the surface of the Earth, these toxic metals can enter food chains and can subsequently cause neurotoxic effects in animals. Rounding out the air emissions of concern are the greenhouse gases carbon dioxide and methane. Excess emissions of CO2 are primarily associated with fossil

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fuel combustion and land clearing, whereas methane gas originates in animal husbandry activities, landfills, leakage from natural gas facilities, and freshwater wetlands. Both gases will be discussed in more detail in a later section on global warming.

14.1 Ozone Depletion in the Stratosphere Far above the surface of the Earth, the stratospheric ozone layer absorbs incoming harmful solar UV-B radiation at wavelengths